I'm a bit torn on this.
We have come to expect that threads will meander, and it's fine. In fact, I suspect a lot of threads would have died in infancy if they weren't allowed to wander.
There are a few threads where it would be useful and important to stay on topic, and/or restrict some types of posts (ie, a PVP fan thread sans the kurtz bashing).
But that would require additional policing by members and work by moderators.
I don't know that making an official designation where the moderators will promise to actively police such threads is really going to buy us anything. When we've needed an on-topic thread, we've simply asserted that in the first post, and reported off topic posts and/or chastised people who posted off topic.
It's worked so far.
Does anyone feel that some threads would be better with such an explicit service?