Heck yea, what a hoot that would be. I can't think of any ways it would empower the caster, IMO it would not be as beneficial. It would however, really be a fun visual, this man always taking his donkey with him, talking to it, pampering it etc.
Familiars are meant to be small animals that can perch on your shoulder. Cat, snake, raven, toad... You can't have a donkey sitting on your shoulder unless you're a big frikkin giant.
However, on the other hand, I would probably allow a multiclass paladin/sorcerer to have his mount be his familiar as well.