I have a feeling this might actuall make a lot of problems worse:

Out of combat, it doesn't change much. Waiting half a minute won't do anything. Most of the worst spells are mainly used out of combat or just before combat.

Also, it would mean the caster could cast one, maybe two spells per battle. What, exactly, is a wizard doing the rest of the time? Hide? GIven that in 3.5, rounds can take ages, this will lead to quite a few bored players.