I'd probably allow it, especially if I started the campaign at level 5 or so (since I have little interest in playing the earliest levels where there's so little to do unless you're a "full" caster). Moon Druid is powerful early on, but mellows out. That said, though, if I did allow it, I'd probably look for little ways to give the other characters their own cool schtick that starts off powerful but levels off. Maybe the Fighter inherited her grandfather's cavalry saber (+1 weapon, minor glamor enchantment). Maybe the Paladin got a holy libram with some rituals in it as a parting gift from his monastery. Maybe the Bard has a special instrument that does...something or other, I dunno, my brain is fried from the Latin midterm I had this morning. Heck, maybe the Druid/Monk (Drunk? hah!) has a special staff that enables the whole "martial arts while shape shifted" thing.
Point is, I'm flexible enough to allow even the cheesy bits...but I'll try to nudge up the people who don't with fun, thematic benefits too. If a rules-as-written option, which doesn't seem to be a huge stretch of the imagination, gets a player fired up and excited to play? I'll probably go along with it. If I know, or have a good idea, that it's really just about being a number-crunched nightmare that "wins" the game all by itself, that's a different story, and I'll talk to the player about that. I'm perfectly fine with doing some charop work to make sure your concept is sound, useful, even powerful; I'm not fine with optimization done to hog the spotlight or to "beat" the system or the like.