I imagine some of them do since it's seen as a 'convenience' for the reader who might upvote the comment (and the article itself, if it's something they're inte

I imagine some of them do since it's seen as a 'convenience' for the reader who might upvote the comment (and the article itself, if it's something they're interested in). And yes I think it does have some kind of social element - people do it in other places and it drifts back into HN. There's a whole bunch of such folklore some of which gets naturally misapplied to HN, 'don't downvote for disagreement' probably being the most famous-while-inaccurate.