The real question in my mind is why so many folks can't hear the phase problems in stereo. If you're set up with your ears equidistant to your stereo monitors, you shouldn't need to switch to mono to hear phase issues. And as far as frequency balance in mono, that should be easy to hear and figure out without making the switch.
As a guy who isn't really a "noob" but clearly isn't an expert either, I think I can probably give a good answer here...
I think it's a great way to "learn" to hear phasing issues because it really calls attention to them. On some level, even without knowing what you're hearing, you can usually tell if something sounds a little "odd" in the mix, but there could be countless reasons for that. If it happens to be because it's phasing badly, then if you collapse to mono, it'll become really obvious what your problem is, and the next time you hear that, you can think, "oh, that sounds like a phasing issue." I mean, I suspect most people when they first start tracking and mixing don't have an innate understanding of what phasing sounds like... Collapsing to mono can help. Furthermore, something badly out of phase is pretty obvious; something slightly out of phase might not be until you train your ears what to listen for.
I'd never even heard the "collapse to mono" recommendation until pretty recently, and for me it was kind of interesting to try. I think over the maybe 10 years I've been screwing around in my bedroom I've gotten a pretty good intuitive sense of how to listen for phasing issues, and ended up learning about phasing more by reading about phase alignment issues than by listening, but it was still kind of cool to flip a mix into mono, listen to it, then flip it back into stereo and listen again and listen to how things change.
I generally do it anyway midway through a mix not because I care terribly about how something collapses to mono, but because it's a simple push of a button in Reaper, so it takes me all of 5 seconds and it's nice to just have an added check in place in case I've done something bone-headed and just somehow not noticed.
I think the thing that's been the biggest eye-opener for me, pun not intended, was starting to mix with my eyes closed. I've been an inside-the-box digital guy from day one, so I've sort of been encouraged to think about EQ tweaks and whatnot in graphical/numerical senses. Hitting play, grabbing an EQ fader with my mouse, closing my eyes, and just tweaking blindly and listening to what I was hearing with no preconceptions of what numbers "should" work was probably one of the bigger breakthroughs I've had since joining this board, if nothing else because at some level we ALL have those mental hangups like "it's cool to low-pass guitars in E below 80hz, because that's where the low E falls," or "a slight cut at 400hz generally sounds good on heavy guitars" or whatnot. It's just the easiest way I've found to toss all that stuff out the window.
I just wish I had better monitors.
