I was experimenting with Mutter, and I'd like to write a plugin in Python. Usually, it's easy to load introspected libs with "from gi.repository import x", but in Mutter's case, I first had to change the GI_TYPELIB_PATH variable to "/usr/lib64/mutter". I think it would be easier for people to find it if the introspection file were installed in the default directory.
It would also make it easier for people who are not writing their own window manager to stumble upon mutter and try unsuccessfully to use it in their applications :-) libmutter is a very specialized component without any API guarantees, so using package-private directories seems appropriate in this case.