An even bigger problem with the scores, and something I didn't find out until after I took the general test the first time, is that because the scores are computed by the CAT software scores at the beginning of the test are more heavily weighted than scores near the end. When you first begin the computer has no information about you so the first third of the test is what determines the bulk of the scores (600, 700, so on), the second third tunes, and by the time you get to the end of the test the questions will hardly affect your score at all. So you can know 90% of the material, but because of the various weightings end up with a lower percentile score because you missed a few questions at the beginning of the test.
Addtionally, while I would certainly claim to have mastered all the math on the GRE, I have not mastered the art of doing it in 1.6 minutes per question. This is especially true in the case of graphical analysis questions, where I have to read the question, look at the computer, look at my paper, back at the computer, ad nauseum just to estimate some basic quantity from the graph. Personally I'd rather get the right answer slowly than the wrong answer fast.
I think the bottom line is that after four years of college we have done enough scholarly work that we should be measured by our achievements and not an arbitrary number assigned by a large corporate conglomerate. If I get an A in upper division math and physics classes (and I do) and less than perfect on the general quantitative section, does that mean I can't do math? Of course, not.