[Pirates]

(f) The pitcher named in the batting order handed the umpire-in-chief, as provided in Rules 4.02 (a) and 4.02 (b), shall pitch to the first batter or any substitute batter until such batter is put out or reaches first base, unless the pitcher sustains injury or illness which, in the judgment of the umpire-in-chief, incapacitates him from pitching.

(g) If the pitcher is replaced, the substitute pitcher shall pitch to the batter then at bat, or any substitute batter, until such batter is put out or reaches first base, or until the offensive team is put out, unless the substitute pitcher sustains injury or illness which, in the umpire-in-chief's judgment, incapacitates him for further play as a pitcher.

Here's why: Sometimes, when a manager makes a pitching change, the opposing manager will bring in a pinch-hitter to face the new pitcher. This is usually to counter a the righty-righty or lefty-lefty matchup. When this happens, what is to prevent the manager on offense from then making yet another pitching change to counter the other manager's counter of his original move? You're catching on, I suspect, to how this could go on and on, back and forth. But Rules 5.10(f - i) prevent this by requiring that the pitcher face at least one batter.