Great! So instead of having one standard way to do video/input multiplexing, we have at least four -- GNOME's, KDE's, wlroots, and weston (and probably a smattering of others). If I want to write a program that works with "Wayland," I'm either going to have to test them on all of the widely-used compositors (because of course they're not all going to behave exactly the same way), or I'm going to have to just punt on them. The former option is 4x the work, and the latter option is me telling users "Hey everone, remember that program that used to run everywhere in every window manager ever that you all know and love and depend on to do your jobs? Well, now it only works on GNOME, since that's all the time I have to support it. Good luck non-GNOME users!"

EDIT: Before you say "just use a toolkit, it'll take care of everything," I can already tell you that users don't care. They only care that the app that used to work in KDE no longer works in KDE. They're not going to complain to Qt or Kwin; they're going to complain to the app author. So the app author becomes responsible for the additional burden of testing their software in a bunch of different compositors, for zero gain.