I think keeping on topic is a good principle to be maintained, as often the original poster OP asks a question but then the thread gets diverted by others opening up another discussion. This often leaves the original question unanswered and the poster frustrated. All too frequently it will be experienced board members who have diverted the thread, perhaps because they know each other (in an online way), have already discussed the issue being raised or want to emphasize a minor or esoteric question raised by the post.
Of course there are those who don't like the first answers to the question and set out to deliberately derail the thread.
I think the moderators should step in and try and keep topics on thread, especially if they are posted by newcomers. It's actually quite daunting to post on here for the first time and we should do everything we can to ensure new arrivals aren't put off.