I've been reading this thread with much interest all afternoon.
Generally I avoid the P&R threads because I'm not religious and I don't feel that I have anything of any significance to post that would aid the discussion. Nor am I especially interested in religious threads, about any kind of religion. I do, though, believe that it's absolutely everyone's right to believe, and therefore to have a space on MN dedicated to religious threads.
However, as this thread is in site stuff, and as I'd also seen the 3pm thread too, it piqued my interest.
I think the invasion of a thread by trolls is always something that should be dealt with swiftly by MNHQ. It happens a lot on the Feminist board, and it's immensely irritating (sometimes pretty upsetting), and if not dealt with quickly really spoils the thread.
But derailment is, I think, very different.
Derailment happens for lots of different reasons, and it's not always a deliberate thing. Sometimes people just go off on a tangent, sometimes a post sparks another thought, which posters wish to express.
Sometimes posters just take a thread in a different direction from that which the OP intended. But it doesn't mean that the discussion is any less valid or interesting.
It is also, of course, sometimes a deliberate course of action, because a poster or posters disagree with the OP or have some kind of personal agenda. If this is the case, then this should be reported to MNHQ - and, of course, other posters can choose to ignore the derailment.
But in general, derailment is a frequent phenomenon on any thread on any forum. I don't really get why it's such an issue. As an OP of a thread, you don't get to control the way the discussion pans out. It's impossible for MNHQ to police posters on the basis of derailment, because defining what derailment is will be completely different on each thread. Ultimately, the only person who can say it's derailment is the OP of the thread - and you don't get to have that kind of control on MN, or on any forum.
If someone is posting in a way you dislike, then it's up to you to challenge that poster, and to argue your point. As long as it's all done without having to resort to personal attacks, then what's the issue? After all, you might end up being presented with information or perspectives that you hadn't thought of before, so you would be learning something. It could, therefore, be argued that derailment can also be a constructive phenomenon.