I don’t think that they are even very bad. They just aren’t… Good. Sure some of the dialogue is awful, but there’s worse. It’s bad enough to keep them from being good, but not so bad that it makes them terrible. They are terrible if compared to esb, but that’s not saying much not many movies can survive that comparison.
I’ll bite…
I’d argue there are MANY films as good or better than ESB. And I say that as someone who for a very long time held ESB as far and away my favorite film ever (some other films have caught up with it in my favoritism but it’s still easily in my top 5 or better). But there’s a reason it’s rarely included on lists like the AFI’s top 100.
Second, the prequels are BAD movies. On so many levels that I can’t begin to understand people who try to argue they’re not. It doesn’t mean you’re a terrible person or stupid or any awful thing like that for liking them. I love lots of bad movies. I have a real soft spot for 50’s B movies and I unabashedly love Godzilla and other giant monster movies. But I can acknoweledge that many, even most, of those movies are bad movies by any reasonable criteria. I can understand why people criticize them or don’t find them enjoyable. Doesn’t stop me from loving them inspite (or often because) or their badness. But I’d never try to argue Plan 9 From Outer Space is some brilliant piece of high art with numerous layers or depth and meaning. THAT would be preposterously silly. Just like that ring theory nonsense…
I am one of those people who will defend the ring theory as possibly not nonsense, for the simple reason that forcing a trilogy to conform to an antiquated storytelling form is precisely the sort of thing that George would do. The ring theory doesn’t make the prequels any better as movies per say, it merely provides a method of interpreting the creative decisions that went into those trainwrecks. Anyone who puts a Yin Yang into a cloudbank is clearly trying to say something deeper than 'there were these Clone Wars, see…'
All that said, the fascinating thing about the ring theory subtext is how this deeper understanding is entirely undercut by the lack of competence on display in the text of the film, so it’s almost impossible to genuinely appreciate this subtext. And it is fascinating, because I don’t know of another series of films that achieves this balance of textual incompetence and subtextual thoughtfulness. Most other films that try this just come across as pretentious.