It gives AT&T less control, and increases the decoupling of a phone number from their service. You're correct that your number is transferable when you change contracts, but you have to change contracts. That is, the number is decoupled at the granularity of a contract. With GV, your number is decoupled from your AT&T phone even when you still have a contract. When someone tries to contact you, it's possible to decrease the likelihood that they will actually call your AT&T phone.

These are also the same reasons why I think it's a great idea.