Some people on HN do not fit your self-reflected view of useful, busy and productive. Reading continuously is a good way to keep the mind sharp and moving even when the body can't do much. On good days I can type a few pages, but other days, I can barely operate a trackball to use the web. In addition to books, code and other tech/science/research news aggregation sites, HN is a real blessing.
I'm happy to say I often spend all day here reading on HN. I'm physically too useless to be a real hacker or entrepreneur any more, and I can't contribute too much to HN, but someone has to flag the spam in the 'newest' queue so the rest of you don't have to see it. I actually happy to be your janitor. It's a whole lot better than not being able to be your janitor, and it's often the only way to say thanks for all the interesting articles and insightful posts.
Thank you for cleaning up.
(From a former "janitor" (not for hn, elsewhere), trying to get more of a life than I've had, as I get well for the first time in my life. An intellectual life is certainly better than no life at all.)
I then ignore the HN tab until I've read through the articles I was interested in (in some cases, I've opened the comments... not sure if that counts as strictly an HN visit). Usually the whole thing takes about 5 minutes including reading the article (I read fast and I'm very harsh at skimming and discarding stuff that had an interesting headline but no substance... working on swombat.com has helped hone these filters even more).
When an article makes me want to comment, I go back to HN, find the article, and comment. Also, sometimes (rarely, I think) I'll get quite involved into a particularly heated discussion and spend a sizeable chunk of time on a thread (e.g. 30-60 minutes). That's a bit compulsive and I try to do that less.
For reference this comment took me about 2 minutes to write. It's on the long side.