Yes, it's a good thing that you don't.

As for the limitation, I'd imagine it evolved as a hunting mechanism. A person is much more willing to accompany something with a familiar face, you know? Taking the appearance of a particularly harmless-looking robin or a second copy of their favorite tea kettle doesn't have the same impact. [he doesn't really know his own weirdass biology exceedingly well though, so he follows that up with a shrug.]

But another arbitrary rule of the pixie dust school of magic works just as well.