If I want to have a roof of half-weather copper that will never change because of lightning, I can just wax it. But if I want a fully weather copper building, lightning can strike it and I can not wax it to make sure it doesn't change back. I think think that there is no reason not to allow players to wax all types of copper, and it might just upset some people who want a fully weather copper road, but can't because it keeps getting converted back.