Last I checked the memory consumption of running xmonad (you know, written in Haskell) on my computer was a pittance; doesn't hog memory, and probably doesn't use enough memory to cause user-noticeable GC pauses.
Though if you're talking about applications in general, and not just WMs; yes, sure.
Yes it is more likely to be applicable to other apps other than WMs, but I also have integration in mind. So what if you want to write a WM like a library that can be used by whatever runtime a user may choose. Not sure that is a good idea but it sure would not be very portable if a GC must be carried inside the library. Memory consumption is not the only issue here.
Yeah, I've been using Xmonad for a couple years and it's just faithfully used very little memory and never crashed. Also, this is across 4-5 different laptops.
Though if you're talking about applications in general, and not just WMs; yes, sure.