It is not a stupid one. The hardest thing in professional sports is hitting a baseball. Hence, when new professionals hit the low levels of the minors, lots of them fizzle out cause they cannot hit the best of the best. Hence, the pitchers have the huge advantage early on. As the hitters mature, they concentrate, as do the better pitchers. By the time the prospects hit AA, most of them have developed some sort of plate discipline and most of them can handle good pitching. This is where the best sort themselves out. There are plenty of top prospects who hit AA and stalled.
At the same time, the AA litmus test is a mark for just taking time. Lots of young pitchers come out of college or high school never throwing over 100IP in a year. Then they come to the minors and are coached, prodded, retooled and tinkered with in a short amount of time. This leads to many injuries. For example, the yankees have a starting pitcher who was a top prospect. Christian Garcia. Did well in both rookie and A ball, but upon stretching his arm out, he got injured. Shoulder, then Tommy John to the elbow. His career is likely sidetracked and may never get off the ground. He never made it to AA.
A wise man once told me (actually a writer for one of the prospect sites) that pitchers in the minors are very hard to predict. Of all the top prospects below AA, figure that 80% of them will either fizzle, get injured, or be nothing more than cups of coffee MLB players. Of those that surpass AA, the numbers approach 50/50 for success (sticking with the big league club) or failure. That is why I like the AA litmus test.