I think they should be separate because when you google for something you're thinking "How do I use XFCE in Ubuntu?" or "How do I use LXDE", etc. and you'd want to read the target page with instructions for that instead of one huge mega list with every window manager on it.
That way we get a nice standalone page for each one (my fave is Unity 2d) that people can just reference to in the context for that desktop. I've crosslinked that page a ton of times and it's nice because it's a specific answer with a simple screenshot.
So whever wants to do this can just fire up a virtual machine install a bunch of desktops, screenshot each dropdown entry in lightdm/gdm and then post/improve one question for each one and toss them in login-manager or something.
Also for some reason these sorts of questions are popular blogspam entries or something, so we might as well do them. :)
