I may not be a sq competitor, but they really are not necessary if you ask me. I honestly cannot tell that their are no rear speakers in my brothers car, and I notice no real difference by fadeing my radio all the way to the front channel. Then again, I never sit in the back. On the flip side, if you only have rear speakers, it just sounds bad.
So in my opinion, no, you do not need rear fill. Adding rear fill could even (while unlikely) cause cancelation and such, so front channel is all that matters to me. Of course, subwoofers are in the back, but they generally fill the car with music anyway, and you cant necessarily tell it is in the back.
But their is no way to change anyones mind, so keep using rear fill if you like it.