My reason with album cover was that many are the art without the title that would appear on the actual cover, so the fact that they're album covers is external information and not an intrinsic property of the picture. I think the other types of covers more often are the cover including the title. But either way, I guess there are examples of both. The ones that are parodies, etc definitely don't fit as meta, which is a problem. You could argue that "art that happens to be drawn for a cover" and "a cover as an object, whether an item in the picture or the whole picture" should be two separate tags, but that isn't likely to happen so if album cover stays general that's fine.
To be frank, I don't care very much about this and I don't want to spend the time discussing it. If someone wants to edit the BUR to match whatever consensus there is, go ahead.