> The only thing that testing tests is test taking proficiency.
This is way too broad. If I give you a written set of the rules of Fight Club, and then I give you a test -- say, I ask you: hey, Wojtek: what's Fight Club? If you proceed to "fail the test" -- i.e., by talking about Fight Club -- I can tell right away that you didn't read, understand, or internalize the rules of Fight Club. Regardless of the circumstance, you will not be invited to Fight Club.
This is indeed a useful measure of something other than test taking proficiency.
This is not an isolated example.
Anecdotally, I cannot tell you how many people I encounter on a regular basis who "fail" such tests. It is very useful to know who fails and who passes these tests.
/jordan