In the half dozen or more shapeshifters I've played, I've never seen a use for partial shifts after getting a first form. And especially not after getting a second form. Horns of the bull is about the only partial shift I use; all the others seem like they are such a minimal bonus that they are worthless when you weigh in the benefit of saving the practice(s) for training later on.

The partial shifts don't seem to raise exponentially with level or skill percentage from what I've noticed.

Sidenote: A form that is resistant to blunt (tortoise) might get some extra mileage from the partial shift edge that gives extra blunt damage reduction while in form.