I love my rearview camera!
It is not a substitute for turning and looking over your shoulder, rather it provides a good view of the blind spot just behind the trunk... which can be very big if you have an SUV. With the camera, I can see my bumper and can even use it to back up within a couple of inches from a wall, all while staring at the dashboard.
For me, it provides great peace of mind when I know young kids are nearby (like in a parking lot). Necessary? I don't know about that... before I had the camera, I would just be sure to back up VERY slowly until I knew the car was in an area I had just seen in the mirror.
I mostly use the camera for parking. The screen superimposes a car-length image that adjusts as you move the wheel - which gives great information on where your car will be, how far from the curb, etc. as you back up.
I thought it was a gimmick when I first bought the car, but it's something I'll definitely look for when I buy my next car.