I am not sure we need to explicitely bump the requirement in the spec. Those bugs quite clearly indicate that the update or the compose is not in sync with the build environment. It's not really the spec fault.

Adding an explicit requirement is easy (if glib used symbol versioning it would even be done automatically), and this avoids confusion if people try to cherry-pick just a few updated packages, so I'd be in favour of having these version bumps as this potentially avoid more bug reports like this one.

I tried with the virt-viewer-0.5.6-5.el6.
If I didn't update the glib2, the error message when I install virt-viewer is:

Error: Package: virt-viewer-0.5.6-6.el6.x86_64 (/virt-viewer-0.5.6-6.el6.x86_64)
           Requires: gtk2 >= 2.20
           Installed: gtk2-2.18.9-12.el6.x86_64 (@anaconda-RedHatEnterpriseLinux-201301301459.x86_64/6.4)
               gtk2 = 2.18.9-12.el6
Error: Package: virt-viewer-0.5.6-6.el6.x86_64 (/virt-viewer-0.5.6-6.el6.x86_64)
           Requires: glib2 >= 2.26
           Installed: glib2-2.22.5-7.el6.x86_64 (@anaconda-RedHatEnterpriseLinux-201301301459.x86_64/6.4)
               glib2 = 2.22.5-7.el6
 You could try using --skip-broken to work around the problem
 You could try running: rpm -Va --nofiles --nodigest

As the result, set VERIFIED

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2013-1577.html