Well for one, when threads go off-topic they have a tendency to devolve into insult and flame wars. That's one of the reasons the forums are monitored. Another reason is to keep things organized for the people that actually give a "hoot" about the topic. For example, if someone actually wanted to see how a war between Hate and Riot is going, they probably won't care about people from outside those tribes bickering back and forth about an unrelated war. I don't know what tribe you're in, but wouldn't you bit a bit annoyed if you were in....say STD, and in a war thread about STD vs. someone else, people kept talking about how the Hate/Riot war was going?
In other words, if you want to talk about stuff that doesn't have anything to do with the topic of a thread, just make a thread. It's rather easy, I promise.
~Morningfrost