The system, of course, if flawed. However, it does give a decent approximation, since most matches within a level are pretty competitive.
One thing about using scores however:
Pete Sampras would never have been rated very high using this system. He would cruise through the set, get one break, and then just serve it out. Many clay-courters would probably be rated higher because 6-1 or 6-2 sets are pretty common with the serve not being as big a factor.