When other people have put a lot of effort into crafting constructive replies to something, and there are no personal attacks on anybody, it strikes me as very poor manners.
Usually there are, however, personal attacks, or moves by certain individuals to broaden the debate into areas where the original poster did not intend to go, and those individuals are well aware that the OP does not want to go there (a common reason might be that he doesn't want to identify certain individuals).
On balance, I think the feature of p p r u n e whereby the starter can delete a thread which has been thus hijacked is a good thing.