Every time i see this picture, i imagine myself trying to shift with that thing-

let see.. coming up to a big hill.
reach around behind me, fumbling for the quick release lever
loosen it, try to shift the wheel forward to get into a bigger gear, and..
bam!
i'm on the ground bleeding from the head while my rear wheel rolls backwards down the hill.

even if you could shift it, i can't figure how you'd keep the wheel centered.