A couple of issues:

Re Mr. Anglim's comment, a suspect issue re. any correlation between GRE scores and success in grad. school is that many people significantly improve their GRE scores by taking courses with prepping companies like Kaplan. So it seems GRE test does not measure any intrinsic ability/talent necessary for grad. school.

Another issue is that the GRE is a form of forced labor: ETS uses one of the sections in the exam (just which section is unknown to the test-taker) as data for future exams, i.e., the section is not counted for the score of the test. So one is expected , basically, to work for ETS for free, producing high-quality data they would have to pay a lot for, or may not be able to produce themselves. GRE also puts out books to prepare for the exams, which cost above $20 each.
Now, ETS could find a way around this by asking, say, "There may be a section in this test which we use as data for future exams: if there is one, would you be willing to take it, or do you prefer to skip it?". This - asking you to work for them for free - is unethical, IMO, if not illegal. And ETS' BS response to this (I called them) is to tell you: "Well, if you disagree, don't take the test." The problem is some programs require you to take it in order to apply for their grad. programs; ETS is the Frank Burns of testing.