I do have 2 cameras ( front and rear ),,,, actually i have one on the R side of my car too, but truth be told i think most situations can be handled with just 1 front camera.
As i see it the main thing the front camera do is proof that what you are doing is within the law, and so if anything happen it must be the fault of another part, or at least tilt the burden of proof to him.
Now he will maybe try to get out of it by lying, and thats when you whip out your footage to proof NO you did not run a red light, make a sudden lane change or stomp the brake or whatever the other guy claim.
My experience using the mobius as a rear camera is that it work fine in daytime - okay at night in a town with ambient light, and on a dark or barely lit road at night all you will see is darkness right up untill the car behind you start to blind the camera with its headlights.
It will more or less be the same with a top of the line camera in the rear window, i use the mobius as its stable - record okay footage, and its small cuz i have my lens mm from the vertical rear window on my little hatchback.
My personal focus is not so much on beeing able to catch licenceplates under all conditions, its a okay marker for the quality of a dashcam, but with current CMOS technology you can only expect to catch a licenceplate during daytime.
Though hit and run is on the rise here, i dont really bother too much with it as my insurance will cover my ass in that case.
And it have to be really really bad here for a accident to go to court, for the most the insurance companies just slug it out among them self.