On this forum we have 15 posts per page. Some forums might have it higher (if there is an option provided in the backend). It might be useful to have a go to top link in post number 5 and below for users who are scrolled down and quickly want to go to top using their mouse. I understand that there is a scrollbar at the right. And most mouse have a scroll wheel. But still a TOP button is still popular and probably relevant on long pages!