Technically, you could also use glib's static library without dependent .a libraries if you link with glib's .a file but do not specify -static; but that's not a good argument against having a Requires for the dependency in my opinion, because it makes the .pc file not work out of the box. Perhaps a weak dependency, but even that sounds strange.
As to glibc-static I think it's fair to say that whoever uses -static will have to install it on their own, so it can be left out of glib2.spec.
Paolo
Although listed in glib-2.0.pc, I didn't find any need for the
sysprof-capture-static package, but it does need glibc-static