Honestly, If you can't look over your shoulder, for what ever reason, should you be riding anywhere that stuff could be coming up behind you anyway; regardless of a rear view monitor or not???? :?
Its crap. I went in to the back of a car because i wasn't looking ahead and in front of me. First objective on a bike look in front. I've also been hit up the back by a car at traffic lights. One of them wouldn't have helped.