a Haiku bot falls into your “triggered by accident” category (any post that is 17 syllables).

Only if opt-out, as the original Haiku bot in the defunct site. OP however made it opt-in, so in order to trigger it you need two conditions - to actively subscribe to the bot and post a 17-syllables comment. The first one won't happen on accident.

a Haiku bot also does not add any new contextual information (it just duplicates a comment).

Arguably it highlights that the post has 17 syllables in a shape that is suitable to build a haiku with, but in general I agree with you. It is not the kind of bot that I personally would inscribe in my comms, nor that I'd use myself.

Even then, a few people like this sort of gimmick, so there's some subjective value for some people. (Certainly not for both of us.)

so I’m asking OP: “why create a bot to spam lemmy with low-value duplicate content, if you don’t even like that bot yourself?”

OP himself answered it - "I wanted to try something easy to learn bot development on lemmy and a few users were waiting for this and so here I am!"

It's a low-hanging fruit, and a few people wanted it.

EDIT: just to make my position clear, I think that a few restrictions on what a bot can/can't do would be great, specially if they come from the admins. IMHO a good bot should have the following requirements:

1. Must be explicitly tagged as a bot, instead of a human being.
2. Must perform a specific, well-defined function.
3. Must only act once explicitly allowed by either the user or the moderators of a community, through a standard approach.
4. Must have a short, succinct output, that doesn't force other users to scroll past a lot of junk.
5. Should be non-prescriptive in nature; it shouldn't be telling you what to do.

Again, I wouldn't use this bot, but I think that it already fits all five requirements.