I'm always in favor of dice being in the hands of the player making the action. It helps to speed play along. I also like Mages being able to roll critical hits on their spellcasting; it allows them the same excitement as warriors. However, running attacks based on Base Attack Bonus harms low BAB spellcasting classes, even if the attack targets "Touch" AC. It seems to me that a different solution is better, unfortunately one that requires a diversion from true d20: A spellcasting roll based on a spellcaster's spellcasting level and his primary casting ability and a defense based on the defender's ability to resist magic (or other threats).
Edit: I went to vote and realized that my opinion isn't reflected, so I haven't voted. Sorry, V.
Edit: I went to vote and realized that my opinion isn't reflected, so I haven't voted. Sorry, V.