Personally, I really don't see the need. Looking at swift-users, the supposed dumping ground, I see only a small minority of threads tagged off-topic (some of which I consider on-topic!) So they're not adding much noise, and some of them have useful answers, even though they're off-topic.
If we did want to more aggressively keep the forums "clean", the work of moving a thread to an "off-topic" category is no less work for a moderator than simply deleting the thread. So I guess, if you do see a need to police things more rigorously, why is having an "off-topic" category better than removal?