I was in the middle of the long thread and then wanted to jump to the first post of it. Scrolling yielded unbelievable delays when "more" posts needed to be loaded, and for some reason I couldn't just jump straight to the top using the slider on the right.
With more long threads or in general needs to jump from first to last (or last to first) post, it would sure become an issue.
There's probably a way to do it with discourse (and if there is -- please share), but it didn't come to me naturally.