It's a numbers game. What is the likelihood of someone winning their case when they rear end you and what is the chance of you winning your case when someone cuts you off and slams on the brakes? Now what if you have a camera at the respective end.
The front facing camera makes much more sense. This is where the insurance fraud aspects come into play. If you can prove you weren't at fault, that's a big deal. Especially if the other party is insisting you are at fault. It stands as a great independent witness. If someone rear ends you, burden of proof is on them.
After some close calls, I'll be buying one tomorrow. Drivers are crazy.