It's true that some tests give little information about the top end of the range -- if your goal is to see whether Joe is smart enough to be a GI or not, then you're wasting everyone's time if you try to separate the top 0.1% from the top 1%, so you don't.
But it's not true that measurable difference stop mattering. I'm lazy to find exact links but the big study to look up here is SMPY, https://my.vanderbilt.edu/smpy/ .
Part of the reason people are wrong about this is that, in many circumstances, they are used to observing a group which has already been selected on some finer measure. The sporting analogy is height in basketball, which is enormously beneficial... some large fraction of the world's 7-ft-plus population is in the NBA. But among NBA players, height does not predict performance very well... because to get hired, they were judged on a much finer metric of actually playing basketball. And the shorter players are selected from a vastly larger pool, and are extreme outliers in whatever other, non-height, talents are helpful.