Metacritic and Gamerankings are tools for game purchasing. It's nice to be able to look at 40+ reviews of a game in a single place. You can then make your decision based on the reviewers pros and cons. The score really doesn't matter, it's just an unfortunate thing that people expect from a reviewer to quantify their opinion into a small package.
My biggest problem with professional game reviewers is that they play too many games. The average gamer would probably take a year to play what these guys play in a month.
I think this begins to skew their opinions. What a game achieves technically becomes more important than enjoying games for what they are. A fun way to relax and escape reality for a while.
Does a little screen tearing or a few muddy textures really subtract from the games overall enjoyment? I don't think so but some reviewers will make it seem like the end of the world.