Starcraft II Coop: The game inside the game.

[Olinser]

Without PvP being a factor I don't see how a class can be too strong. The need for balance is much lower, if you don't like a particular commander you just don't play that one.

Because they design the tougher Brutations based on perceived player power.

If a couple are significantly more powerful then it either ends with people playing those characters facerolling them, or being near-impossible unless you play those characters.