The fact that every discipline uses the same test for admissions says more about the quality of academia than the scores.
The GRE is a pretty generalized test. Most undergrad programs use either the generic SAT or ACT as their test factor - it's a efficient and useful way to measure a large group of people. The most important prerequisite for most graduate programs is an undergraduate degree in that field. How you performed (as well as the quality of the program you graduated from) is likely the most useful tool, so I can see how the GRE would be downplayed.
In some graduate programs, specifically law and business, what field your undergraduate degree is in isn't that important. I'm guessing this is why the LSAT and GMAT tend to weigh more in admission decisions than the GRE does.