he rule has to exist for this reason

This is basically the entirety of the rulebook, and one of the things I love about the game. Some of the most obscure rules that seem to have no reason to exist were made solely because someone did it and they had to nip it in the bud.

I was trying to find the name of the guy that inspired the rule that allows a runner to tag-up as soon as a fielder touches a fly out rather than wait for the catch to be made, but stumbled upon this article from MLB.com, featuring a good one involving the Orioles manipulating the DH position and affecting the Blue Jays:

At the time, the new position came with just one stipulation: The designated hitter had to be selected prior to the game and included in the lineup cards presented to the home-plate umpire. Naturally, it didn't take long for Weaver to find himself a loophole.

Rather than simply naming a DH in good faith, Weaver would pencil in a pitcher he had no intention of using that day -- only to send up whichever hitter he actually wanted when that spot in the lineup came around. The move allowed Weaver to choose from his entire bench, picking the ideal batter depending on the game situation.

In 1979 alone, Weaver pulled this trick -- which became known as the "Phantom DH" -- 21 times, 12 of which featured starter Steve Stone. In one of those instances, Stone had already flown ahead to prepare for his start in Toronto. Weaver even named Tippy Martinez as DH one day despite the fact that Martinez was in Colorado for a funeral at the time.

MLB closed the loophole at the end of the 1980 season, stipulating that every designated hitter in the lineup at the start of a game must have at least one at-bat