Media matters claims to be a site about correcting "mistakes," or reporting of incomplete facts, made by other news organizations, not a site for editorials about supreme court nominees.
In the case at hand, media matters had an opportunity to correct the record about the ample precedent for appointing people to the bench who did not have prior judicial experience. However, they did not, instead choosing to author a piece on Miers that corrected nothing and was largely matters of the writers' opinion.
The site is showing its true colors as nothing but a site for dedicated liberals to find "hit pieces" about republicans.