It was successful enough in the past that people did it. It costs effectively nothing to post comments, so if you get anything, even a little reward once out of 1000 comments, it's a gain. You need HP, but you can still use your HP for voting or whatever else (as long as it doesn't use too many RCs).

Or for that matter, even if it isn't quite all that successful but shows enough promise (occasional rewards), people may do it, which is still harmful. It's not something where you lose a lot of money by being wrong, you either make some or make none, but either way spam up the chain, cause annoyance on the social layer, and increase everyone else's cost for RCs.

As for the other mitigations, yes that could work, we can try that if needed.