I have plans to buy a 4-channel amp in the future to power both fronts and rears. I don't want to hear the rears, but want them there in the event that I have someone sitting in the rear. I agree that rears are not necessary for front seat occupants.
Powering rears off the deck isn't always possible, it seems my Kenwood head unit doesn't supply power to the rears when my rear line-outs are connected to my mono sub amp.
So, the reason I want a 4-channel amp for fronts and rears is twofold: I don't want to lose the ability to adjust front-to-rear fade and I'd like the fact that the amp provides a non-fading summed signal line-out that I can pass along to the mono sub amp.