It would be interesting to know how many users use Empathy. I don't myself, and maybe I'm not representative, but I don't think it is that many - Pidgin seems more popular among people I know.

I don't follow the /usr bit - what does mounting /usr have to do with loading or not loading WebKit?

WebKit is around 24MB, which is as you say quite hefty, but it's even worse as it depends on some other libraries - sqlite, libpng, etc.