Music only considered, I think most of this has to do with the overall quality of the recordings, 2 ch vs multi. I think it's clear that there's vastly more high quality 2 channel -analog and digital - content and, thus, purity of signal would likely flow through quality 2 channel dedicated setup, sans some multi-channel post processing of the signal..all things equal. If the a hugh percentage of musical source content was simply superb hi-rez, well recorded content on multi channel, as status quo, then I think things would be different. So quality over quantity as a priority, every time, IMO!
Then, whether either scenario is or isn't ever the case, one still needs a high quality setup for all the loudspeakers and corresponding seating and acoustics in same space to take advantage and maximize the sound quality from the setup and speakers! - this is NOT typically the case, I find, from most multi ch setup's, let alone those who don't know how to maximize the sound quality from a basic 2 ch setup! It's just harder to do right, fundamentally.
So, 2 ch is easier to setup and get good sound by default, and better source material in 2 channel mix, equals advantage to 2 ch !
...I still prefer dedicated multi mix for movies, however,..digitally processed in an AV pre/pro out to quality amps and setup, is hard to beat for that medium.
