Yes, well, Windows 8 allows users to step in and out MetroUI as they wish at any moment in less than a second. So, depending on usage patterns, what you say may or may not be the case.
So, as a consequence, I can only speak for me here, of course: I basically never/very rarely end up in the start menu/ModernUI when working on desktop / traditional laptop. When I do, it is in those rare times when I have to start a specific desktop application that I decided not to have a shortcut on the task/app bar for. While on the "desktop" mode, the issue of viewing one program at the time is not there as it is not with 7, or XP.
Also, I can hardly imagine in which way one is getting in and out ModernUI so much, buy hey, if those people say so... I'm part of another set of users who operate on their windows 8 based machines differently.

In addition to that, using the new ModernUI start menu not only does not bother me, but actually I find myself finding what I need quite faster then when I had to dig into the old start menu, so the ModernUI is up for just few moments, it's not something that gets in the way of anything your're doing, as I read in many critical articles on this topic.

On a tablet instead, one would spend a lot of time, maybe always for entire usage sessions, in ModernUI, and that's where the two-apps-visible at one time limit would kick-in. I don't have any experience with Win8 tablets so far, so I can only guess that would be as ok as, or better than-my current experience with Android/IOS tablets, which do show one app at the time.

Actually, I can't understand why, while having a full featured desktop OS available, MS did not push a bit further MetroUI to display 3 or 4 apps simultaneously. Or, better, I understand that the paradigm there is ONE main app + ONE side app (a messenger, a task list, and anything that can fit in just one column on one side. But allowing for 3 columns or 4 quadrants would have added two more more interesting layout opportunities compared to standard current tablets.



Oh, I just had to work in UX for some projects.

Again, in usual Win8 desktop usage the same typical Windows paradigms stay, so there's no change there.

Using horizontal real estate and scroll, which is more typical of ModernUI (and also depends on the App within the ModernUI context), is intended for touch-screen devices, so that's an addition to, not a replacement of, typical desktop paradigm. And it is intended for a completely different usage pattern (whether one likes it or not, I mean). So, I can't see where this criticism coming from, actually. It's not that Windows8 forces you to use MetroUI in a standard desktop or laptop. I'd rather say the contrary, actually: you can easily stay away from it if you so wish.