If you bought a compute game, let's say Command & Conquer: Red Alert, you would expect the Allies and Soviet Unition militaries to be balanced, right? No obvious power disparity between them despite the fact that both sides have different approaches to warfare, so if you're playing a 4v4 multiplayer, and choose Great Britain while your allies choose Mother Russia, you won't be dragging the team down by playing an underpowered faction.
Even if the other players compensate for the Great Britain's weakness by agreeing out of game to not make you feel useless or overshadowed, you have to admit it would be, at the very least, a bad decision on the part of the publisher to release a game with serious flaws that should have been picked up in playtesting. Which is something that I'm glad Westwood Studios didn't do, because that would have be horrible.
Also, being the one the rest of the team has to play down for cannot be a good feeling, from what I can tell.
Why not just restrict casters to the 'set list' spellcasters? Healer, Beguiler, Warmage and so forth?