Tadpole powers (and the emperor) being restricted to the evil path would at least offer something for the evil path itself. Because right now you just lose out on companions, npcs and quest content if you side with the goblins (and the absolute). And you can use the authority & tadpole powers on the good path with no repercussions, since the narrator warnings don't mean anything. At the same time even if you don't use them, there is no way to avoid the emperor. Even being openly hostile to him doesn't change much (except it makes even less sense for him to stick with the protagonist, instead of trying to convert one of the companions to his side).

That was what WotR did with the mythic paths (except for the late game paths), tried to offer different content, and it was not just good against evil. Aeon and trickster were law vs. chaos on the cosmic scale, for example.