Of course there is the matter of screen real estate. The Model 3 has a smaller screen and has to display the stuff from the instrument cluster as well, so I’m not sure there’s enough room to have a useful rear camera display as well as a useful nav. At least the Model 3 doesn’t have the rear window visibility problems the S does.
Compared to my Garmin Nuvi or my iPhone SE running the Google Maps app, the Model 3 screen has plenty of real estate to show a useful map alongside a useful rear-view camera. Though it would be nice to be able to use one of the steering-wheel buttons to switch back and forth, since I don't need to see the map constantly, but I'd rather not have to reach over to the screen every time I do want to see the map.