How useful GRE really is?

As I am setting up for another round of practice tests for GRE, I am really doubtful about how useful all this time investment is going to be after I have been admitted into a graduate school. I could have been using all this time to read up on various books on cognitive science but something like GRE is deemed more important than how much knowledge one has of a particular subject that is going to be part of his/her thesis in graduate school. If GRE is all about analyzing how flexible and adaptable graduate students can be in different challenges, then one might as well give IQ tests rather than studying up for GRE. At least IQ tests will be more fun and useful to prepare for.