I wouldn't go with a web cam for rear view.
2- Wouldn't cut it under low light conditions.
3- For best performance, you need CCD not CMOS based camera, only stand-alone true camera will have CDD.
4- You have to have your carpc on before you can actually see the picture, that is any where from 45~90 sec of delay. Actually, you have to have a PC to use it.
5- A dedicated camera is almost on all the time.
There was an excel ant post here the other day that covered the subject, look around, it is helpful.