I was taking my CPR recertification today, and the instructor was telling us how quickly a child's condition can get bad when he/she starts choking. She used the example of a child in a car seat eating candy. With front facing seats, it's easy to check on kids in the back with the rear view mirror, and some vehicles (i.e. minivans) even have a wide angle mirror for keeping an eye on more of the rear seats.
With rear facing seats, I would imagine that you can't really see what your kids are up to, and if you have only one child in the rear facing seat, you may not know if he/she is in trouble before it's too late (i.e. no second child to call for help). That's why I was thinking it would be a good idea if there was an option for a rear seat camera that could be monitored on the 17-inch display.
Disclaimer: I don't have children (only nephews) and I'm not getting the rear facing seats for my Model S. I do know how my sister likes to keep an eye on her boys when she's driving them around though.