I rely on mirrors to know what's back there. To me a backup camera is just an irritant, as in the corner of my eye I can see something changing on the dash when putting a vehicle in Reverse.
Yes, I'm getting better at ignoring it, but don't want to lose the instinct of paying attention to things the dash is trying to tell me. Also, the backup camera is another reminder of something I really don't like; the touch screen.