Also certain types of power tools for mac probably need use Quartz directly as well.
Also certain types of power tools for mac probably need use Quartz directly as well.
Honestly I think this is a pretty fair approximation of "no one". How many people are writing tools like this vs. the number of people writing regular applications? A very small number, I'd say.
And after working extensively with both libX11/libxcb and libwayland-client directly, I can say that none of them are particularly pleasant to work with. Actually, no, that's not true: libwayland-client wins, easily. Every single Wayland protocol has code generated for it that works exactly the same way. I suppose the same is (more or less) true of libxcb, but libX11 (and all the other libraries you might have to use, like libXrender, libXrandr, libXext...) are a complete mess.
And even then, libwayland-client has a much lower number of concepts you have to understand than libxcb does, simply because the Wayland protocol has a small number of concepts you need to understand. libxcb is definitely an improvement over libX11, but it can't magically make all the underlying X11 protocol concepts become unified.
Why would I want to add more complexity to a display server?