i'd like to know as well.. but because I want to go the other direction..
backup camera in the nav screen to me is pointless, I don't look down at my radio when backing up, I spin my ass around and use the rear window and my outside mirrors for the corners, the only help the camera is to me is it allows me to see the tip of the rear bumper so I know how close i am to climbing my truck up the hood of the honda civic behind me.
in the rearview mirror would make more sense to me, because I use it, I don't look down at the nav screen..