Of course there's no reason to read reviews if you've already decided to see the movie. But a lot of folks are in the position of trying to make that decision, and reviews can be very helpful.
Rottentomatoes.com is a great site in that it posts dozens of reviews of each movie, and comes up with a percentage of positive reviews. They also have a separate rating for "Cream of the Crop" reviewers, generally the bigger name critics and publications. (BTW, Pirates is doing worse in Cream of the Crop than in the overall average, while that is reversed as to Cars). So you can diversify away the risks associates with the idiosyncracies of your hometown paper's reviewer. Whether you agree or disagree with any or all of the reviews, it's certainly a valuable service.
As for the box office, the bad reviews make it seem much more likely that Pirates will have a big dropoff after what's bound to be a huge opening weekend.