I think it's largely a feeling of "what will they remove next"?

Say an engineer is a a daily user of ten particular GNOME 2 features. GNOME 3 removes four of them, and hides three more. At that point, the engineer is wondering what will happen to the rest when GNOME 3.3 comes out.

In contrast, environments which are explicitly intended to be customised very rarely disable or remove features. Someone who's been using the same configuration for ten years can know that if it works in release N, it'll probably work (maybe with a few minor tweaks) in release N+1.

It's worth noting that XMonad has many more features than Metacity; it's not bare-bones by any means. Converting to it isn't like converting to a minimalist WM.

The thing is that Gnome3 is relatively highly integrated, and contains implementations of various functionalities. Using a 3rd party tool tends not to work well, because the Gnome implementation conflicts.

Using a bare bones window manager doesn't cause the same issues. It's not as integrated, but you can mix and match parts more easily.

I see it as risk avoidance. The learning curve with the basic WMs is steep but a one-time cost. With richer WMs, You pay a tax with your time and productivity every time the UI designers f* with your s*. It seems like UI designers these days are about making BOLD MOVES without apologies.

Look into the Mate desktop - it's a rebuild of Gnome 2 with maintainable code. Best way for a test spin now is get the Linux Mint Debian Mate iso and try it in a VM. I actually like Unity, but in Mint the Mate desktop is my favorite, looking forward to Mint 13 with it as a default.