I find myself downvoting (flagging) them when I see them as submissions, and I can't think of any examples where they were good submission content; but for comments? There's enough discussion where the AI is the subject itself and therefore it's genuinely relevant what the AI says.
Then there's stuff like this, which I'd not seen myself before seeing your question, but I'd say asking people here if an AI-generated TLDR of 74 (75?) page PDF is correct, is a perfectly valid and sensible use: https://news.ycombinator.com/item?id=46164360