But that's a tautology, isn't it? You should test when you're ready -- that makes perfect sense. But how do you know when you're ready? Well, by the testing criteria! So, the way that you know you're ready for the test is if you've...already...taken the test?
The rationale really isn't hard to understand. Training time is an arbitrary standard that does not perfectly reflect the skill and readiness of the person to whom it is being applied -- I don't think USAF has ever said otherwise. But those who focus on the imperfection of that standard can never produce an alternative that's any better. So, it's what we got -- not perfect, but also a long way from useless.
of course you are right about tautologies - I guess for me this one is lessened by (a) the possibility that you fail a test, so you may have been wrong, and (b) all sorts of formal and informal social arrangements that ensure that (a) is not too frequent, and not too disheartening when it happens.
I still dont see how training time should have a decisive or even prominent place there, if skill is what is tested. There can be such blatant discrepancies betwen training time and skill, can't there? I am not even trying to argue a point, I have really never quite understood. Is the point that aikido is so "holistic" that technical skill is only part of what ist tested?