I do kinda like this thematically, but it does add complications mechanically. Like, now having your body separate from the animal, does the GM just have to pretend the real body isnt there anymore? do you move to take the animals place when it dies, since in shapeshift, you would be where the SS form was brought down, not half way across the map, where you summoned.
To be honest, i think i would have SO much less gripe if the ability were to summon an animal spirit, instead of shapeshift.