I'm with endarire in that if you want to allow it, go for it. I never find myself needing to give very strong buffs to my druid players, but YMMV. However:

Quote Originally Posted by Carden-Gix'oth View Post
Sure, sure. But they still gain bonuses to Saves (Good Fort/Ref, Poor Will), BAB As a druid with the same HD, and Feats. So it's, for all intents and purposes, the same as leveling up.
Well sure, if you ignore everything that is different, then what you end up with is pretty much the same. It's actually not the same though, with one of the main differences being that AC's don't get to pick a class to advance in. the just get bonus animal HD.