I believe, right now, that the backs are being utilized in the order of which they pass block. Chris Warren is clearly the most violent of our backs, and Carter is the shiftiest. Porter sees the field because he knows how to pick up a blitzer.
I further believe that Shane is double, triple, and quadruple-checking the coverages before utilizing his check down. With as porous as our OL has been, I'd love to see Warren run a center screen or two. It would, though, involve Shane throwing into a pile of bodies (almost all of them should be his teammates, but that's the risk).