If you are a professor at a research university you have probably spent time on graduate admissions. How good is the GRE as an indicator of candidate quality? Is the subject score more useful than the subject score? What about relative to undergraduate GPA? Similar questions apply to the SAT and undergraduate admissions.
In both cases the answer is that standardized tests have roughly as much predictive power as GPA (SAT is about as powerful as HS GPA; GRE similar to undergraduate GPA). Not bad for a brief test! When these factors are combined the overall predictive power is increased. My opinion is that standardized tests load more heavily on cognitive ability and less on conscientiousness relative to course grades, hence the non-redundant information in the two measures.