The "margin notes" are usually ten times more interesting to read than "discussion threads" where few people try to convey their points back and forth.

That sort of argumentation is generally just boring to read, lacks value, and happens between the two or three people involved. As such, it doesn't really contribute much value on a public internet forum and I usually skip all that stuff myself.

You can sense if a comment is written for no one in particular. It washes away all the need to argue, and it humbly gives to each reader what they're willing to accept from it. This can be a lot or nothing but it all depends on the reader, too.

I write comments with a similar idea in mind. I feel that if what I've written might give one person an idea, point of validation, sense of similarity, or a moment to seriously think about something for a while, it's worth posting. I of course do not know beforehand whether this happens or which year it might happen if it does. I rarely write as a direct response to the parent, and if I do I tend to eventually diverge from the "reply mode" to "writing mode".