What happens if you delete the last class that #imported something? Do you remove the #import? What if the #import wasn't automatically added? Still automatically remove it?
How about if you don't need the #import for a class, but, instead for something else (say, it #imports a bunch of other classes for you instead, like UIKit.h)?
The ideal, for me, would be the same as ReSharper: a plugin that adds functionality, but that won't happen with Xcode any time soon.