I don't think poisoning things has anything to do with it. It's honestly just a filler move since Scarf Exca is pretty bad anyways and doesn't have a lot of options other than that move set.Excadrill has one viable move and that's poison jab. Poison jab helps it to poison strong walls like slowbro, cresselia, and porygon2 eviolite. You should run defog on latios or latias because when u rapid spin and people notice your scarfed, they can set up much easier to sweep your team.