I agree with the idea that INT doesn't need to go up to 7 schools versus 1 for STR, but I have a different idea.

I think Polymorph should use Constitution, not STR. Polymorphing affects your body, so a high CON should allow for better control (and longer-lasting Polymorphs). Also, tying polymorph to CON makes it more usable for many different character types, not just warriors or mages.