Well, it's a good thing you can have 6 Pokemon in a party, so when Swampert is in battle, and you see your opponent is switching into a grass type, you can switch out into somebody who can take care of grass better than Swampert. Also, the OP said that he is breeding for all of these Pokemon anyway, so if they really wanted to have a Swampert that was made to destroy Grass types, they have the option.