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