It's fine as long as you stick to a subset of things (in particular, you'll find that gnome/kde/lxde/etc. allow widgets to be separate processes and use a separate widget manager, but enlightenment requires them all to be managed by "e" — otherwise they'd play poorly with its virtual root.

There is, by the way, one rather obvious (hm, ok, obvious *if* you know a little history) reason to stick this in the server rather than the window manager that I should have mentioned before.  Let's say you are using a virtual root window manager and have a large desktop, *and* you for some reason switch the X server to a video mode that requires panning (consider needing to swap the monitor, and the new one doesn't support the resolution(s) you were using; this happened a lot back then, since there wasn't much consistency in video modes between monitors, X servers didn't read DDC information (and only high end monitors provided it back when this was added) so you needed to specify mode lines, and as a result the only common video mode was often VESA 640x480).  Now you have both the server and the window manager panning, and the interactions between them made the result *very* difficult to work with:  get the mouse pointer near the screen edge and *both* of them pan, resulting in a much larger than expected shift; and in extreme cases (that 640x480) what you were trying to reach might well end up off screen in the other direction, or close enough to the screen edge to trigger *another* double-pan that leaves you right back where you started.  I think this is the original reason it got moved completely into the server; since the server needed to support panning anyway, having both server and window manager do it made no sense at all.