Shells can't weild a Keyblade. He can. Plus, I don't really think he is a shell. I think that he could possibly be something like a shell, but not exactly. We know that he has to be someone's 'shell though'. One of Ansem's Reports states that in another world, a "you other than yourself is born. A creation that begrudges both light and dark..". That implies that someone in Twilight Town (by someone, I mean multiple) being were created that could weild the power of both light and dark...But not in a manner of conjuring up the light and dark powers as elements. Simply....'molding' it to their will.
Basically, I think BHK is his own person, but not his own person...If you know what I mean.