Because if you play your hand right on a fighting caster, you can have near-infanite SP. Add in the fact heals are faster to top me off, and that i don't have to switch to a wand, and im moving through the quest faster. Nevermind the fact that its always nice to not let your party go down. I doubt your gunna pull out a wand mid fight to keep the other guy from going from incapped to dead.
Oh, and another thing. You cant res without burning alot of scrolls. That sucks.