I don't think that there's much structural difference between an article and a comment (apart from one sitting underneath the other; the "depth"). In terms of rewards, upvotes on comments give author rewards and curation rewards just like upvotes on articles. So there's no more or less point in upvoting a comment than there is to upvoting an article, if that's not too existentialist.

Although having thought about it again I suppose upvoted comments don't go on the trending page, so the upvote doesn't give them exposure. Maybe they should. Maybe there should be a trending page for random comments. I would visit that page.

I tend to upvote comments, particularly if they can improve on something in the article I've written, or if they generally add to the sense of discussion or community, or if my fingers get clicky. Or sometimes just to thank people for stopping by. It's never very much but I think it can add up and help to keep people rolling. Look after the pennies and all that. Maybe I will do an analysis to find out. Are there people that just comment. And how much do they make? But not today. Today I must write a story about an island.