I don't see what harm it would do. It's just like having "off topic" in your post, only an entire thread. Further, those who don't WANT to see the off topic discussion would not find it at all - they would avoid it.
I do not see the purpose of having an Immune Board. After reading all the rules of this forum, I realised that this website was created specifically to encourage "intelligent discussion." Having an Immune board, wouldn't that be contradictory to the rules of the forum?