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". Problem is some programs require you to take it in order to apply for their grad. programs ; ETS is the Frank Burns of testing.