I've written about this issue before, but as usual, David Poland says it much better than I ever can:
"...the Number One reason for the diminishment of the critic has nothing to do with the web or bad decisions by editors or box office chatter… it’s marketing dollars. Period. End of discussion.
Twenty years ago, spending $50 million marketing a movie domestically was virtually unheard of… and now it is fairly average for a major studio movie. When Disney’s Armageddon was marketed domestically at a cost of almost $100 million in 1998, it was front page news in the Wall Street Journal.
That kind of money blanketing awareness of and interest in a movie is inherently more powerful than reviews or any other publicity tool. It has nothing to do with how many “bloggers” are writing about a movie or when. It has nothing to do with the quality of the coverage or the criticism. It has to do with human nature.
People have used critics as arbiters of their choices because quality criticism was one of the few tools available to a civilian in making a choice about a movie or other ticket buying choice. And people still build relationships with certain critics whose opinions they either agree or disagree with, but rely on to offer them insight into what buying choice they might make. However, they have more content that is not opinion-based than ever. They see more ads. They have easier access to trailers, long the #1 determiner of choice in theatrical ticket buying. And while they used to wait until a day or two before a movie to read criticism and/or features in their local media, they now have a treasure trove (or slush pile, depending on your POV) from which to choose before the local paper lands on the front porch."