Off-hand, I can only think of the reason why I wouldn't want infinite scroll and that is because I can navigate forward and backward by page without losing continuity. Especially the 'New' feed (95% of what I read), which is continuously updated, the page URLs keep the posts confined to that particular page of posts regardless of however many new posts have come in since I started clicking to other pages. I think infinite might be okay on the main feed since they tend to be a little more 'permanent' (ordered by their rank)