Prepare to repel boarders...
I think you are going to get as many answers as members, it also varies by the type of post.
If someone is asking a serious question, say "my airbrush is doing XYZ and it never did that before, what should I be looking for", and people randomly come in and start dropping Monty Python references and jokes, that is kind of rude. Of course if he is a jerk, a complete knee biter (like insulting people who start by suggesting he check very basic things) then he probably deserves to have his thread go in the dumper.
The threads that start off kind of jokey or at least are easily percieved that way (Should I take out a loan to buy the new $400 1/72 Spuce Goose, or my kid busted my models, should I kill him) should expect to go in the dumper by page 3. You can't start a silly post and expect it to only stay with your brand of silly.
Personally I don't mind a little unintentional thread jacking here and there in real threads, you know someone asks a question about a 1966 Mustang (a car ) kit, some one posts a model they built with that kit to show how theirs came out, someone then comments on the model shown, someone comments on the comment, and the next thing you know the thread has moved to how the Mustang got its name, then moves onto the air war over Korea, and then leads to a new thread setting up a post war jets GB, these things happen.
In fluff threads I really don't care where they meander, some are just silly fun and some become interesting discussions.
Its really up to people using their judgement on how disruptive their comments are about when its time to start a new thread or keep at it. If the OP is walking down the new path I'd say that is a free pass to continue.