Not trying to rekindle the spice/gas wars here, but I just thought of one reason that spice is good to have on a street. Piggies! As you may or may not know, piggies can be nourished when someone harvests a tree while they are nearby. No need to feed those piggies anything else if the street is populated enough. Piggies eat spice. Piggies don't eat gas. So if there are piggies on a street and the only trees are gas trees, I think you'll have to feed them other stuff or else they will die prematurely.