On reddit, I collapse conversations to show myself that I've read as much of that thread as I want to, not that I have any particular problem with it. I'll do it for a variety of reasons, the conversation strays into a topic I'm not interested in, or gets too technical. For me, collapsing is a positive, it means I read the thread instead of just passing it by.

While I support introducing collapsible threads, I don't think they should have any weight on the ranking.