Have you seen the kinds of tripe that people leave in comments?

I saw meaningful comments. Don't sure what you propose... there is "tripe" on HN too.

Varies wildly depending on the blog, in my experience.

I set comment threads to close on my blog because after maybe a week the only submissions I get are attempts at spam.

My experience is different. I don't receive a lot of spam with Disqus. If I disable it I receive 10s of spam posts everyday.

I prefer to leave blog threads open since I don't know when people will find my sites and contribute. Except for posts really connected to a specific event, technical posts has a larger life.

Same for me. I've got topics 12+yrs old still open on Disqus (the topics started before disqus and comments were imported when I switched). I get very little spam (knock on wood)

I dislike closed comments though sometimes I have to remind people to look at the date of the post :-p