As Nifft Said, if I want to make a druid/barb hybrid I don't necessarily want to be able to cast spells in Wildshape, I want to rage in wildshape, I can cast spells when Im in human form.
That said it wouldn't break anything to give it to druids for free at some point.
As for its comparison to fighter or wizard I think the key point is that wizards and fighters both get bonus feats to further customize their characters (as well as increase their power). One of the biggest things 3e dropped the ball on is allowing some classes to be massively customizable but restricting most classes to rote "paths" at best (via how the character chooses feats/skills). I hope every class has bonus feats (if only 1 or 2) in 4e.
Technik