Just to pile on, I think there is some irony in this. How much of the music that audiophiles enjoy on their objectively well-measuring systems was mixed and mastered on ATC, PMC, etc. ( in other words "snake oil/inferior") monitors?
I do believe in objective measurements and blind listening, but A) objective measurements only tell you what you are measuring for, such as flat frequency response and low distortion, and B) blind listening tests can only show you what you are listening for, in many cases the "best" sound of a recorded piece of music (best meaning balanced or accurate).
Honestly I don't know exactly what those qualities are that lay bare the flaws in a recording, as Spocko said, but if hundreds of mixing engineers say that ATC gives them this information I wouldn't be so quick to dismiss it. Maybe it's not a ruler-flat frequency response and ultra low distortion that helps a mixing engineer get a good mix, but rather a bump in a certain frequency range is beneficial, and maybe some amount of distortion or "low bandwidth" also indicates when a recording may sound harsh or muddy on a large swathe of commercial playback devices.
I can get a good sounding mix on a set of well-measuring Genelecs but when I play the song back from an iPhone speaker I can hear immediately that the hihats are way too bright. I'd never want to mix on an iPhone speaker alone, but if there was some magic set of monitors that was able to present a full frequency range and also point out those flaws, well that would be the ideal monitor for me as a mixer. I wouldn't care how the measurements look.
There's also the matter of fatigue. I may not want a completely flat response above 8khz because after 15 minutes of EQing cymbals my ears will be shot for the rest of the day.
If someone could answer why some monitors work better for mixing than others I would love to know why, but I don't think it can be answered by simply saying people are brainwashed by marketing.