Originally Posted by
Aquillion
I mean, while I don't like how it plays, the problem it is trying to solve is obvious and it does do a really good job at that.
In 3.5e, a lot of the brokenness of casters comes from stacking spells beyond what the developers envisioned or intended. Battlefield control wizards and CoDzillas operate by combining spells in ways that effectively provide multiplicative boosts to their effectiveness - two battlefield control spells that individually just slowed enemies down but which together make enemies totally ineffectual; or a big pile of buffs that individually would have made the cleric or druid a bit better but together make them vastly stronger than the fighter.
Balancing every spell when combined with every other spell is hard, so it's much easier to just say "you only get one big spell at once."
Of course, the downside is that finding those combinations is part of the fun of the game, even if it's potentially unbalancing... and concentration blocks you from combining spells that aren't game-breaking together too, which is just annoying.