I don't know any more than you, but I believe they say there are relatively few DVDs currently encoded for discrete 6.1. The extra channel or two must be drawn out of the 5.1 soundtrack of most discs. They also say there is relatively little info that comes out of the rears in a 7.1 setup.
My question would be - with QS8s properly placed who would feel the need for 6.1 or 7.1? Pretty enveloping as it is, IMO.