Of course, just thought I'd clear up any possible misunderstanding.
As far as minimalistic, I wouldn't know. If you don't use any other programs that use them, then I guess they are unnecessary and overkill (he's just doing rectangles and monochrome graphics, after all). But if they were already being used, and there was a way to create a monochrome buffer and use it in the wayland compositor without converting it to some other format (or, even if you used the standard buffer format), then I think they would be fine.
However, I'm guessing he would like to assume his users are using the minimum necessary, in which case they may not be using apps which use cairo, etc..
This is most likely there because pkg-config depends on glib. For all Cairo dependencies, they're listed in the README.
Cairo does not depend upon glib. Pango does.
Anyway, I think some of you are missing the point. Most of rendering code (for pixman) is just plugging FreeType's output into pixman's glyph cache system. In fact, adding a rendering backend that used Cairo instead would be roughly the same amount of work.
Additionally, Cairo does not deal at all with Wayland, requiring you to support EGL and shared memory buffers manually (doing both buffer creation and fallback yourselves). I believe that Weston's toytoolkit is the only other thing that does this for you at the moment, and it is, as the name implies, just an example of how a toolkit could look. Even Gtk+ currently only supports Cairo image surfaces with a SHM buffer.
Hopefully this clears up the motivations and intentions for this project.