Standardized tests are a great way to do a cross-group comparison of skill sets.
The first time you give them.
After the first time, you are no longer primarily measuring the skill set. You are primarily measuring the test prep. Because motivated test takers will always beat the test...and it will become easier to learn the test than the material, roughly by the second round of the test. Insofar as only a few people do test-prep, you might still have a decent test. However, once any reasonable number of folks participate in test-prep, as per all the IT certifications, No Child Left Behind, etc...then what you're really measuring is mostly (~50%) how much time the student spent on test-prep.
If you are trying to compare between groups that have identical levels of test-prep, then again, the tests become useful, but only with potentially high sunk costs. So what does the SAT/GRE measure? Mostly, it measures a combination of parent and student motivation/discipline if you're willing to do test prep. If you're not willing to do test-prep, or you opt not to, then it measures IQ pretty well.
Now...what ought colleges to be looking for? Ought they to be chasing folks who are disciplined test-preppers? If they should...then the SAT should be weighted highly. If they shouldn't...then perhaps not, though it is a much better predictor of college success than high school grades. The interesting questions to me is what happens when one group reliably uses test prep at levels far above what other groups do? What is the sane thing to do when looking across groups? What is the right thing to do? Are they related?
I think those are the interesting questions that need to be addressed around Murray & Unz. I think their claims need a broader audience, but they're highly non-controversial among the HBD crowd. "Is this news?"