The longer a thread runs, the further it gets from the original topic. As a general rule, if a thread title looks interesting I'll read the first two page max, then stop, safe in the knowledge that most of the subsequent posts will add little if nothing to the subject. Threads meander, it happens

However, people who regular 'hijack' threads - quickly taking the focus away from the original query or topic onto a very tangentially related topic (or one not related at all) should know that it is considered very bad form in the world of the interwebs.