Sure, I agree conversation flows and evolve. But blatantly off-topic posts that come out of no where and trollish derailments are also counterproductive.
Why leave them in the thread?
because along with weeding out 'troll' posts, regular posts get trashed. it happens with the best of intentions, but no one can be so perfect as to know every nuance and meaning of a post. Maybe the post has subtle intent, is a relevant pun but you just didn't get it, or maybe the post is just answering a previous post, and, while that isn't clear to you, the intended recipient would understand it....
when one plays 'conversation cop', one tends to go all 'rodney king' every once in a while.
use the ignore function and it won't be such an emotional and physical drain on you, having to wade through all the 'troll posts' that ruin your important threads.