By the time you find a thread and the number of comments reached > 20 it becomes hard to navigate. Any new posting is difficult to gain attention.
Do you agree? I see e.g. here at HN
Reading like that is something I've developed over time as I'm active on this site for more than 5 years now. I wouldn't recommend doing it, unless you think you might want to try it for fun. That's always a good excuse :)
For commenting myself on heavy contested topics, I comment on a parent if it's popular and my comment to it is related. When the parent isn't popular, I sometimes comment on it, depending on how relevant my comment is and how low it is.
I think comments > 10 especially nested ones are discussions benefitting mostly the folks in the discussion. Its difficult to get a summary out it.
I like to think about nested comments as joining an hour long class in the last 10 minutes.