It's important to understand that playstyles are not off/on, all or nothing. You don't either have a completely defensive style or no defense at all. Like the one "true" style, all players tend to use a mix of different playstyles at some point even if they mostly use a particular style.

Playstyles are like emergent mechanics. And like mechanics, playstyles can be designed to create a balance against each other. The potential of interactions between all playstyles is what I consider to be the "game" of a video game. 

For the purposes of gameplay balance, the following are questions that I ask myself when designing a game or balancing an existing game. 

• Which playstyles do I want to promote and which do I want to limit? Most games are designed to promote aggressive, action playstyles and limit defensive playstyles. When games are designed the other way around, high level competition can become very dull like a stalemate where both players are trying to react to their opponent yet neither will make the first move. Designers put a lot of work into their games and a common sentiment is that they want players to experience and use as much of the game's moves/mechanics as possible. Why bother giving each fighting game character 30+ moves if players only use the 4 best ones?
• Personally, I can't stand it when players excessively avoid fighting and confrontation in a fighting game attempting to run out the time to default in a win. I also don't like stalling and static battles. I love interplay of all kinds and dynamics that change the battle conditions. I design games with well rounded mechanics and design, and I prefer it when games are design to encourage players to use more well rounded playstyles. 

• Does the viability of a playstyle decay with time? With use? Or in some other way? One way to keep certain playstyles, mechanics, and options balanced is to incorporate a degree of decay. So even if a player elects to play to the extreme of a playstyle, the decay will reduce the effectiveness of the playstyle thus encouraging the player to play more conservatively or switch to another playstyle/strategy. Being able to cancel out of attacks with a FADCancel in Street Fighter 4 gives players more ways to be offensive. This would greatly unbalance the offense-defense design of the game if not for the decay design. FADCing takes 2 bars of one's super meter. With 4 bars total, a player can only do a maximum of 2 FADC in a short amount of time. To earn more super meter, players have to fight. With this design, even the most aggressive player can't play at their maximum effectiveness all the time. 
• In Halo, ammunition is the primary source of decay. You have a limited amount of grenades to throw and bullets to shoot. Even though the Sniper Rifle is one of the most powerful weapons in the game, its ammo is very small. Also, grenades are good for keeping opponents back or flushing them out of hiding. You can only hold a maximum of 2 grenades of each type. Once you're out, your ability to defend and compromise other's defenses drops. 
• In Super Smash Bros. a shield covers your character's entire body. Shields also wane gradually on their own. When your shield is attacked it shrinks even more quickly. Because you can only stand around and shield for so long, even the most defensive shield can be countered. Also, Smash Bros. Brawl has a feature called stale-move negation. Basically, the strength of your attacks decay with repeated use. So even if you want to only use a few moves for your offensive playstyle, soon you won't be doing very much damage or knocking our opponents very far. As Sakurai, the creator of Smash Bros., says... "Let’s face it, it takes more skill to use a character’s full move set than to rely on the same attack over and over." Sakurai is clearly a designer that promotes a well rounded player. 
• In Advance Wars, units in a battle will consume ammo when attacking. However, they also consume ammo when defending or counter attacking. Because counter attacking is automatic, you can effectively use your units to burn through some of your opponent's ammo. With little to no ammo, a player's offensive playstyle possibilities are reduced.  

• How much skill is required to effectively use a playstyle? Generally, it's much easier to defend in a video game than attack. In Street Fighter all you have to do is hold back or down back. In Smash Brothers holding a button puts a shield around your entire character. In StarCraft you can wall yourself off in your base with cannons or other defensive structures/units. In Advance Wars, you an build units and stand on your own property for a defensive and healing bonus.
• In Mario Strikers Charged, a soccer-hockey hybrid type game, each goalie is an AI programmed to defend the gaol. So, even if the opponent refuses to touch their controller, the goalie will defend the goal. In this way it clearly takes more skill to attack/score goals than to defend. 
• In some games, it's hard to be very aggressive. In StarCraft, even if you have the superior forces, chancing after your opponent as they run/retreat across the map is not the best use of your advantage. The more units you have, the harder it is to organize and use them effectively simultaneously. As a game carries on, the scale of battle increases making all playstyles more difficult to maintain. 

• Can one playstyle be countered effectively by another playstyle? If so, how do the skill levels compare? Let's say that we can accurately measure skill on a scale of 1-10. When designing your game, you have to consider how much skill it takes to be effective at a playstyle and how much skill it takes to counter one playstyle with another. If one player has a 5 skill level and plays offensively, do you want to design your game so that a 1 or a 2 skill level defender can easily stop or counter the level 5 attacker? In general, the more skillful the execution of a playstyle, the counter should require an equal to higher amount of skill. 
• In Super Smash Brothers Melee, shield grabbing (a technique that goes from defense to offense) is very effective for blocking and reversing aggressive attacks. It's so effective, that it can completely shut down beginning players. Shield grabbing is a very easy technique to do too. Just hold the shield button and hit A after the attack hits your shield. To overcome shield grabbing, one must learn how to space (aim) their attacks very accurately or L-cancel to speed up their character so that they can attack/escape before the shield grab lands. Both of these counter techniques take a lot of skill and an aggressive playstyle to be effective. The good part is, once you can effectively space and L-cancel, only opponents with a similar skill level can keep up.  
• In Advance Wars: Days of Ruin, on some maps an opponent can use a very specialized playstyle where they spam inexpensive yet relatively powerful Mech units and slowly flood the map. Though these units move slowly, they can effectively combat Infantry and vehicle units. To counter this specialized playstyle (without air/sea units), you can employ a long range defense. The Mech units will take too much damage from long range as they slowly move in. Once they're softened up, you can use backup units to pick them off. This counter playstyle/strategy not only effectively shuts down the Mech units, but it also builds the veteran status (strength) of your units. To be successful with this counter style, you must recognize or create a choke point and solidly support your strategy. It takes patients and resolve, but it will succeed. Thus, countering the annoying Mech unit spam playstyle requires a more skillful defensive playstyle.

• How do extreme playstyles increase or reduce the amount of interplay per unit of time? Even if you support a variety of extreme playstyles, you must consider what happens to the "game," (interplay) of the system when they come into play. Different combinations of playstyles stress different parts of a game system. But what if a defensive style is so solid that there's nothing an opponent can do to crack the defense or gain any kind of advantage? What if an offensive approach is so effective that after the first hit, the target is at the mercy of the attacker until the match ends? If you value interactivity, gameplay, and competition, then it's important to make sure that one player can't effectively remove control of the game from another through their playstyle. 
• In Smash Bros. Brawl, grabbing the ledge of a stage gives you temporarily invincibility. Some players with extreme Playing to Win playstyles love to hit their opponents once and then run to the ledge to abuse the invincibility repeatedly. This is called planking. Some characters are so good at planking that the opponent can do virtually nothing to gain the advantage. So considering this extreme case, an entire fighting game can be reduced to a hit,run, and wait exercise. If I were in charge of balancing Brawl, I would eliminate planking by adding a degree of decay to the ledge invincibility. Doing so would allow players to use extreme ledge defensive playstyles, but not for very long. 
• In the Legend of Zelda: Four Swords Adventure multiplayer battle mode, there's a level in which a player can create an absolute defense. By climbing to the top of a ladder and shooting out continuously, it's impossible for any other play to climb up and do anything about it. If a player decides to do this, the game is at a stalemate. Situations like this should be prevented. 
• Back in the red/blue days of competitive Pokemon, a few Nintendo sponsored tournaments I went to prevented players from putting to sleep/freezing more than one of the opponent's Pokemon. Without this rule, the game would be reduced to a "snooze fest." Other tournaments banned excessive uses of the move double team and minimize because these moves greatly stall out matches when used excessively. If such a rule isn't in place, both players can make their Pokemon very hard to hit with the majority of the game's attacks. 
• On a more personal note: As much as I love meta games and how emergence and competition can evolve a game into something that far from what you originally thought the game was capable of, I'm more of a fan of variety and balance. When games evolve too far past their original design as far as the most viable competitive strategies and playstyles, there's always the risk of warping the game. Character, levels, mechanics, tactics, and strategies can all be completely overshadowed by discovered, emergent, and refined playstyles. From a designers point of view, the closer a metagame stays to a game's core design, the easier it is to keep the game balanced and the playstyles well rounded.  
• For example, from what I understand of Mario Strikers Charged, the chip shot is an powerful mechanic with a wide range of applications for scoring goals. It's so effective, that competitive players have found that chip shots are more effective than normal shots on the gaol. Chip shots are even better than fully charged goal shots. Also, the players who can make the most of chips shots are the fastest players. To compete, not only do you have to have to learn these chips shots, but you need fast characters to chase down your opponent's fast characters/plays. Thus a high level game contains few to none of the following core elements... medium-slow characters. Charge shots. Items. Mega strikes. Super moves. Defensive dekes for avoiding opponents. That's too much quality design to lose just because of a single playstyle. 

Unbalanced and unfair playstyles are just like unfair tactics for competitive multiplayer games. They're like deadly viruses. Here's what David Sirlin has to say about the balance of unfair tactics and playstyles:

An oversight in a single player game that makes the game too easy to win might only affect 1% of players. A balance oversight in a competitive multiplayer game can ruin the entire development effort. An unfair tactic will travel quickly throughout the playerbase like a virus and ruin the experience for everyone. Game balance is a potential “single point of failure” for the entire project, so get it right.

In the end, I'm the kind of gamer, competitor, and designer that loves the challenge that players face while expressing themselves through their playstyles in a back and forth battle-debate of sorts. There's nothing like a deep, balanced game and head to head competition. I respect the player who will sit down and fight against me any way they see fit (without cheating). But I respect the designer who creates a balanced system even more.