Over on Thought Balloons, Kevin asks "Do Reviews Matter?"
As part of his post, Kevin somewhat answers his question by way of Roger Ebert: That reviews serve to bring to attention those good works that would otherwise be passed by.
He then goes on to ask the corollary: If that's the case, why bother to review popular titles that everyone has heard of?
Towards an answer, here's my take: By reviewing popular works, the reviewer allows the reader to gain a perspective on the reviewr's tastes, biases, etc., with a work that they've both encountered. This allows the reader to make a more informed evaluation of the reviewer's opinions for works that are unknown to the reader.
In other words, I review something like Batman so that people will know how to judge my review of Gotham Central. Because I'm stingy with high ratings, I hope that people will notice that when I give a book a '4', that really means something.
It's not a question of reviewing one type of thing or another, but rather that a reviewer should review both.
That's just my take...