Interesting question... Maybe we can get some discussion going on this one. Here's my take on it - it comes down to the specifics of your system and real-world vs. theoretical. Yes, equal power to all five (six? seven?) identical channels would seem correct assuming all speakers are indentically rated. That's a big assumption.
In my system, for example, my mains + center are rated at 86db SPL, the surrounds are 89db SPL (Snell SR.5). Therefore, it will take considerable less power to get equal SPL's from my surrounds relative to my mains. Snell provides a chart on their website for determining how much power you will need to do the job in various scenarios, and for my room size they recommend 40-60 watts is plenty. I have found this to be the case.
My receiver was made prior to Dolby Digital, althought it has 5.1 analog inputs, (thank God!) Therefore, it has unequal power. It is rated at 90 WPC x 3 up front and "only" 40 WPC for the rears. It was standard practice in Pro Logic receivers to have less power to the surrounds. Guess what? I still have to trim the DB's down slightly to get them to balance with the fronts in my processor's speaker setup. The more efficient surround speakers more than make up for the lower power they get from my receiver.
Even at very solid listening levels during movies, I do not notice clipping or compression in the surrounds on Dolby Digital. I know there is the potential in DD/DTS to have a soundtrack maxed-out in the surrounds, but in the real world this does not happen very often and then only for brief bursts. Most of the time they are coasting along providing immersive ambiance. I have all speakers set to "small" for movies with an active sub handling the LFE even though my front mains are full range floorstanders.
Would anyone care to comment if equal power all around would benefit my system (or one like it), assuming sane SPL's, and why? Please, no receiver bashers, I know upgrading to separates would improve everything overall vs/ a receiver. I'm asking specifically if EQUAL power would be a benefit in my case. Cheers, JZ
