Cant' argue with that reasoning IF the speakers are much smaller. I use identical speakers for all 5 channels in my HT system (5 Totem Mani-2's), and drive them equally using the Theta Dreadnaught amp (400wpc into 4 ohms), which I have found to be the ideal solution (perfect tonal balance). In your situation, if you have a processor that can adjust output level by speaker, you could pump up the rears a little higher if needed...
Surrounds don't need as much power
I'm in the process of upgrading amps and wondering if the surrounds really need as much power as the mains and center channels. The speakers are smaller and there's less information going back there, it just seems logical that less power is really needed.