I don’t mean to pile on or anything, but I’m not convinced there’s much validity to this either. The way I see it (again, I’m coming at this from a hearsay perspective because I haven’t actually had a chance to really do it yet) is that gutting the lows and highs (and maybe even the stereo image) forces you to focus more on the arrangement, which is arguably where the most important mix decisions take place. With good monitors, I can hear the vocals pretty clearly even if there’s a million other things going on. On crappier speakers, it all turns to mush, and forces me to go back and say “maybe that 3rd piano is a bit much.”
So I view it more as an arrangement tool than a mixing tool, but since arrangement is one of the more important aspects when it comes to mixing, by association it becomes a good mixing tool.
Again, all hearsay. I may find it to be absolutely dumb. I may love it. I have no real opinion on the matter yet.