I think I would have to agree with aeghrur and RDK on that matter. It seems like as long as a tangent still has some relevance to the initial topic, then it is worth debating. After all, we can't expect to solve every issue all at once, sometimes smaller issues that deal with the bigger picture have to be resolved first.
Basically I think if two or more people are debating something that is tangential to the topic at hand, but is still somewhat relevant, and they are debating well, they should be left to it, and eventually it will conclude or branch off into another tangent (which may be too far off topic by that point).
As for the topics in the PG not being debate-worthy... well, they are being actively debated (or at least some of them are), and as far as I'm concerned, just about anything is debatable as long as there are two sides to pick from.