> (why is composition the job of the wm anyway?
> What does laying out windows have to do with making them look flashy?)
Both of them deal with windows and I assume that's a large part of the code. If separate WM would be controlling the window stacking, that of course affects how they're composited.
For example Matchbox window manager offers a library for window management, I guess there are also other window managers that do this. Window management code can be hairy, but it's stable, it doesn't change like what people want to do with compositing. -> Makes more sense to have library for window management than for compositing.