To an extent, this is what a vancian [i've never heard it called that before, but that seems to be the term all the cool kids are using] magic system is there to prevent - your mage can cast invisibility if he has it prepared, but you thief can be stealthy at will. The mage has great power and versatility, tempered by moderation - if you prepare that transformation, that's one less fireball. If you cast it now, you can't cast it later today. You have great power at your disposal, but you need to be careful where/when/how you use it.
That's why I like that kind of system, and I'm sad that it seems to be fairly unpopular even here, because going to something like a mana-based system means that either mages need to be nerfed to the point that they're far less fun to play, or they're so overpowered compared to the other classes that it's silly to pick anything else. I don't think I've ever played a game where the mage worked as well as in BG2 (not BG1 because you spend too much of the game so pathetic that you're almost useless, nor ToB where the mage really started to outstrip the other classes too much).