I guess that one incentive to change it from earth to poison damage is immersion and logic. Although not everything in a fantasy world needs to make sense, the idea of 'earth damage' is horrendrously flawed. How does earth damage you? It can't, unless it's either poisonous or through inertia, which essentially translates to physical trauma no different from a club. There is nothing inheirently dangerous about it, unlike fire, cold or electricity.
