I think people are giving too much credit to gorilla glass, and especially to the kevlar back.
Gorilla glass can shatter, but what some RAZR owners are seeing is the screen under the gorilla glass is cracking and shattering. And the kevlar is not as protective as you would think, it multiple layers it is, but with the thin sheet on this phone it won't absorb any shocks and drops that could damage the internals.
I prolly wouldn't get a case if all I was worried about was scratches, because they're no big deal really and the gorilla glass should do a decent enough job protecting the screen. But if you're worried about dropping the phone, then you should get a case that does a decent job of absorbing some shock. If anything to make sure the screen under the GGlass doesn't crack or spider-web.