It is true that a good engineer will do most of the mixing on a much cheaper set that gives a real world sound (Mackie's, Sony NS-10's, etc) but the high end speakers are there to point out any flaws in the recording, etc. Let's just say I've never heard Dark Side of the Moon sound so bloody good before or since!
You mean Yamaha NS-10s. Sony haven't made a decent pair of speakers for 40 odd years. The last 'good' speaker they made was a re-badged B&W.
Also I would like to point out that once you get past about £8000 on speakers, the changes could not be described as for better or worse. In all honesty, my favourite sounding speakers cost £10,000, and yet I have heard the B&W Prestige speakers set up connected to the same source (they cost £17.5k each) and I preferred the first pair. It's all a matter of opinion past that point.
You can get a pair of incredibly neutral almost zero distortion speakers for £3k, but what scientific instruments detect isn't what we're interested in. We're more interested in what our ears hear.
For example, recently I mixed a live recording (32 channel multitrack) with my little B&W DM303s. Really about the cheapest decent hifi speakers you can get, nothing amazing when compared to the kind of speakers you find in many studios. They aren't even particularly neutral, but I know them very very well (they were my first proper hifi product). I'd done a mix previously with some KRKs, and the engineer agreed with me that the final mix sounded better having now been mixed through my DM303s by me (not wanting to blow my own trumpet, but I'm the best sound mixer in the west
).