Ooh, its crystal clear now! Thanks for explaining this MDS. It all makes perfect sense now... I understand...
Okay, so basically the upmixer just works in reverse, only rather than the easy job of removing something that is already there, it has to "guess" what should be there and place it. (Just kind of saying stuff out loud here.)
I already knew it did that, but when you state it like that it makes perfect sense.
Given that, I'd say lower-end systems benefit from the guesswork more than higher end systems. The lower end systems would have more to work with, thus creating a "better" sound, while the high-end systems can already reproduce the audio perfectly, and thus don't need something to fill-in, or "guess" for it. My theory is this is why you elite gurus don't think upmixing helps any, while my less-than-100%-perfect system can still stand to gain by it. Does this sound logical?