It wants to scale from a full fledged desktop to a small scale phone. Hence some stuff might look duplicate but will benefit certain scenario's. I can get into that. So, this is Wayland trying to be the same as Mir (or better) I guess... .
Wayland has the full blown complexity, but because it is built on extensions, this complexity is optional (and extensible).
But wayland is just the protocol that clients can use, facing them you need a shell + compositor (the compositor is the one implementing wayland, but it also has impact on the shell).
Weston is the reference compositor for wayland, and although it is made to test functionality, it is quite usable (much like the Xorg server). It also has a mechanism to implement shells as Weston plug-ins. For example, Kwin will be implemented as a weston plug-in, but GNOME shell and Enlightenment will provide their own compositor. There is also an IVI shell plug-in, that is there to easily make OS environments for in-car devices (and also follow more closely the development standards of this industry). And then, there is this effort (for Weston) to provide pre-made functionality for cases where you don't need an environment at all (single app), and don't want to package the full gnome shell with you.
"No, KWin doesn’t “use” Weston. KWin is currently not yet a Wayland compositor, but can render to a Wayland surface instead of an X Overlay Window."
Martin Gräßlin, http://blog.martin-graesslin.com/blo...-frameworks-5/
So they use it at the moment, but will eventually provide their own compositor.