When a game's story and ambient development is influenced by player actions, cheaters can bake a big dent in the game overall. Hell, player groups can do that without even cheating.
I remember Eve online. They had this in-game/out-game currency; a card that gave 1 month playing time. These could be bought in-game, but since the market was influenced by players, a serious group of them started pressuring the game's market in such a way that these monthly-visas became too valuable, and completely useless in their original purpose. Whomever wanted to extend their game time, had to pay real money in the eve store, because no player could earn enough money in-game to pay for the monthly-visa cards.
Now this was done with game mechanics, no cheats. But it still resulted in a kind of cheat and unfair game-play.