Throughout the course of Doctor Who, whether it be New Who or Classic Who, there are clear favourites and least favourites regarding the villain or alien the Doctor faces. Majority of the unpopular aliens are just one-offs - (for example, the Abzorbaloff). But sometimes overuse of an idea can also play a major part in whether you like or dislike an episode's villain. So sometimes, when a much loved character or villain is overused, they should not go away forever, but take a holiday until is it necessary to bring them back. But the biggest reason to why a villain shouldn't return is based on the portrayal in their most recent episode. If they lose all factors of what makes them a good villain during an episode - (which has happened) - it immediately lowers expectations and makes episodes much less enjoyable.