With my iPhone 4, I wouldn't think it is necessary to have a back protector but with this one, there's so many people complaining about the aluminum part scratching (scuff-gate anyone?) that I think it's necessary. Unless you use a case with it, cuz then the back is protected anyway.
Yeah I know, it will be interesting to see what my phone looks like in a few weeks. I'm using a photive bumper, SPG screenprotector, lucky labs wraps, no back protector. The bumper does have some lift though.