Wasn't the question about pitchers? Why would you bring up position players?
I stated sidebar. My comment is secondary to the OP, but related in the sense that players have to let it fly to show what they're capable of. This stuff always comes up when kids are going to showcases and camps.
Back to the OP
Which is often why a "showcase player" isn't necessarily a gamer. Coaches can sometimes be fooled, too. I remember my son went to a college camp and the coach made it a point to tell the players and parents that they were releasing a couple of current players that showed well in showcases and were recruited on that basis, but it didn't translate to being a daily D1 player. Are you really "capable" of whatever it is you show once with maximum effort? Technically, yes, but I don't think that paints the complete story of a player. I know a player who is 92 across the diamond (absolute hose of an arm) who, because he knows he can "sling it," often takes an extra step or time to throw the ball, which has forced max effort throws that cannot be handled by the first baseman when they are off-line or in the dirt. In this case, all that velocity became a crutch and now that players are faster he has to figure out he needs to make other adjustments in order to be a reliable defender.