It annoys me too. But when the OP is getting a real kicking, and people are being nasty, I kind of get it (why they want it deleting.) I think personally, that MN should have a '7 days only' page, (as they have a 30 and a 90 days one,) so they can move the thread there, so people can keep chatting on it. Because tbh, I rarely see threads, where the OP is 'identifiable.'
The only time there is a good reason to zap it, is if the trashy gutter press feature it in their rag. Then again, by then it's often too late, as the (online) newspaper often screendumps the comments and posts them, so they can still be seen, even when the thread has gone. Also, the headline will often give it away.
What REALLY irks ME, is when someone posts a thread about a dilemma they are having, and multiple dozens of posters come on the thread, with advice and help and kindness (amongst maybe a few harsh comments,) and there is lots of helpful, interesting advice and tips (and links to other websites that will help,) and MN discover the OP is a previously banned poster, and ZAP the thread.
I have seen posters spend an hour or two on one thread before, advising and reassuring the OP, and giving out helpful advice; just for all the comments and useful information to disappear into the ether!
Extremely annoying. If it's a previously banned poster, can they not just ban the poster, but leave the thread up?!