In terms of engagement, a user is much more likely to bail every time you require them to perform an action like click "next page." On Discourse they can just keep scrolling down as long as they care to read.
It also helps to track their position. On long topics, users often won't read it all in one sitting. I am a member of the Something Awful forums, and since they use vBulletin with pagination they can only know if I've opened a page and they use that to mark the whole segment of 20 posts as read. What if I stopped half way through? What if I opened the page in a new tab then closed my browser without looking at it? They assume I've read it all!
Logically, what is a "page" anyway and why should a user care about it? What does page 669 mean versus 670? How about we not expose implementation details to users and just let them get on with their reading?
