I believe the rear-facing camera would get much more action from inside the car than from the rear. The system would have to make white balance choices, after all. Plus, depending on the car model, there wouldn't be much to see outside from that distance.
The other day I was parking and looking at the screen on my car showing the rear camera for parking, and I thought: why this only works for parking? There should be a system in the car to allow recording as a dash cam, and also a way for us to look at it even without engaging reverse. Rear cameras will be mandatory starting this year here in Canada. How hard would it be, and how costly, to create a recording system also integrated?