The absolute least worst outcome is they chew up your token budget. But what tends to happen, and this is much more serious, is they poison future work and make further modification of the codebase more burdensome and error prone.
Seeing LLM comments in other peoples code is very upsetting because theres just so much meaningless noise.
It usually only takes one of those in the context for the model to reorient its behavior for future edits.
Agreed.
Probably better to add whatever instructions it takes so that the agent doesn't write comments, at all, ever. If you need comments to understand the agent's code, the necessary information should already be in a conversation somewhere; and you should summarize it yourself, because the comment will be for your own benefit. Otherwise you are letting past-agent steer future-agent more or less at random.