I'll have to place myself in the no pre-buffing camp after all. I did feel the frustration from time to time, that I had to wait for the combat flag to pop up before casting certain spells. But rather that than this constant buffing ritual before every fight that happen for example in BG2.
When it is an necessity to pre-buff before every major battle it remove both some fun and strategic elements from the fight, and if it isn't necessary then the spells are probably not worth their spell slots.
As other has mentioned, it is about choice and options. Do I waste some precious seconds in beginning of the battle to buff up my squad, summon monsters or do I go all out with the artillery?
When there is no consequence, as in pre-buffing, there is just the question of: Do I pre-buff and probably win the fight, or not pre-buff and probably lose it. Because if pre-buffs exists, the battles will be balanced for it as well.
Thou I would like to see the restrictions loosened a bit, out of my head I can think of shapeshift, cipher charm powers (?), some character centered spells and maybe summoned weapons.