Along with it’s synonyms, like “it’s so bad it’s good”, “laughing at it, not with it”, and also the only variation that got discredited everywhere else too, “enjoying it ironically”.
It’s not that the idea behind these statements doesn’t make sense. It’s easy to imagine why a bad show could still be enjoyable. I’m not personally interested in that kind of enjoyment, because I like to take my fiction seriously, and if I’m not even able to do that at all, I just stop watching it. But in principle, it’s easy to believe that someone would like laughing at the sheer absurdity of such a horrible show getting made.
But have you ever noticed, how the entry level for that excuse is much lower in ongoing works’ fandoms, than with standalone works? With standalone works, it’s really limited to the most painfully bad ones, such as literature with misspellings in every sentence, or movies with painfully weak special effects and practically nonexistent acting. With serials, it can be just something like “a manga with an unrealistic, over-the-top setting” or “a generic harem anime”. What’s the difference? Maybe with standalone works, people with slightly unusual tastes can fit in by watching something, enjoying it, and when it turns out to be unpopular, join the chorus about what a waste of time it was, but with serials, they need an excuse to keep going.
For example, in the current season, we have the generally disliked shows Guilty Crown, Symphogear, and Highschool DxD. There are some people who dropped them early, some who still genuinely like them and call them good, and some who completely dislike them, but feel a sense of duty to finish them. And then, there is this fourth category, people who believe that they are bad, but want to continue watching them because they are “so bad it’s good”. Isn’t that kind of strange? Even if someone found their badness amusing at first, but couldn’t sincerely “care” about them, what would make them going back to that specific show every week? What would keep them hooked on, in a way that they often describe to be “like watching a car accident in slow motion”? After all, why couldn’t they just forget about it during the week, or start watching one of the other bad shows instead, to also mock that one’s premise?
My second problem is limited to the phrase “guilty pleasure” alone, and it’s unique issues. Namely, the way it is sometimes used to imply that the show is really good, but the viewer is so deeply intoctrinated with the public opinion, that they can’t fully separate their own from it. That can range from anime itself being a guilty pleasure, for people who take mainstream’s stereotypes too seriously, and see themselves as “creepy pathetic otaku”, to certain subgenres, or shows, that are disliked in the fandom, being “guilty pleasure”.
That is usually based on arbitary judgements like how you are not supposed to like mindless eye-candy, or how you are supposed to prefer moderate stories over camp and over-the-top ones, or exactly how “realistic” shows have to be.
That’s probably the worst thing about that phrase. The suggestion that some other people know better than you, and that you should feel guilty about liking something. That entertainment has any other objective requirement than entertaining you. When I see examples that use it that way, I’m not even angry at the writer, but everyone in general.