I don't see it that way. Yes, a sloppy field will hamper the cut, but it will also hamper the defender's ability to engage the runner if he does manage to cut as much as he can. Think of the line of scrimmage as a equals sign. Mathematics proves that an equation has to be equal on both sides of the sign. Rain doesn't affect just one side of the line of scrimmage - it affects both sides. If the field were dry, and the runner cut on a fly sweep, wouldn't the defender's ability to engage after the cut be better on a dry field? Yes, rain diminishes abilities, but it diminishes abilities on both sides of the ball. The amount of TD's might be less, but it will be less on both sides. Whoever was going to win on a dry field should also win on a wet field.
However, I will concede that if a team's offense is so shallow that it relies only on fly sweeps it's not much of a team. I don't consider the Pirates to be that shallow of a team.