I'm with MrT on this one. At the point where further upgrades to my 2ch system produce vanishing improvements to timbre, then I'd consider adding mch. But I never seem to arrive at that point. Admittedly, the qualities of timbral accuracy and spatial presentation do occur in different planes of the listening experience. Listeners may prioritize one over the other. But in a 2ch system, upgrades that improve timbre also tend to improve spaciousness, whereas one should not be deluded into believing that the simple addition of more channels improves timbral accuracy. Of course, the overall gestalt of music is what counts, but I disagree with Eldartford that it is relatively easy to obtain correct timbre.
Mig007, regarding Teresa Goodwin's Pos. Feedback contributions, her oft expressed opinions about the inferiority of one source format versus another are dubious, as her points are made purely from the perspective of a budget system. Similarly, the point in your original post regarding superiority of mch is relative to the quality/price of the system. Moreover, it would be important for me to assess any mch system based largely on how it sounds when switched into 2ch mode-- as needed for the majority of LP/CD/SACD sources. And on this point, the majority of mch rigs may fall flat on their faces relative comparably priced 2ch systems.