Zest, I took that advice, and bought a pair of M2s for the back surround position, use QS8s for side surround. I listen to music in 7.2 and movies in equal measure. As to music, most of the time CDs, but sometimes SACD or DVD Audio. I have my M2s up for sale. If I had it to do over again, I'd get QS8s for both the back and side surround positions.
My front surrounds used to be a pair of M60 like Michaura M665s, but I moved these gorgeous towers to my office and picked up a pair of Thiel CS 3.6s for fronts, a heavily modified Merak (Axiom made) center speaker with a pair of Axiom 6.5" drivers, a little surgery and a Reference 3a de capo mm tweeter and cross over (the Axiom drivers run free). This is the best sounding center speaker I've heard, although I will be building a slightly larger Axiom shaped center cabinet for a second center using a pair of Axiom 6.5" drivers, my remaining de capo tweeter and x-over, and a supertweeter.
Anyway, sorry I rambled off there ... my recommendation, based on actual experience (at least with the recommended QS8/M2 combo) as opposed to some theoretical model, go with QS8s for side and back surround duty.
Enjoy your system!!
Enjoy the Music. Trust your ears. Laugh at Folks Who Claim to Know it All.