Let me see if I can explain this in such a way that Rob can't possibly take offense ...

In the Soviet Union of the 1950s and 1960s, the process of making movies was controlled by the state. The government's definition of "a good movie" was "a movie which accomplishes social good." Thus, the efforts of communist movie-makers were directed not so much at entertaining the public or providing a moving emotional experience as they were at encouraging people to work hard, stay on the farm, and show respect for the government. When everybody else uses the term "a good movie", the "good" modifies the "movie". When the Kremlin used the term "a good movie", the "good" meant "a movie which can accomplish a greater good." The result was a long series of Boy-Meets-Tractor type movies which seemed, to everyone but the government which was producing them, to be a waste of time and money.

What we have here is really the same problem. When everybody else uses the term "a good fastball", the "good" modifies the "fast". What everybody else means by the term "a good fastball" is "a ball thrown really, really fast." Rob, however, has overthought the problem, and is using the term to mean something else. What Rob means, I think, is "a fastball which can be used to good effect in winning the game" -- in other words, a fastball which accomplishes a greater good. The best fastball, in his mind, is the fastball which has the maximum impact on the pitcher's chance of winning the game.

There is nothing irrational about this; it just looks stupid because he's using the term "a good fastball" to mean something different from what everybody else uses it to mean. So what I am saying is, Rob really isn't stupid; it's just that he's a communist.