Well in most forums, it will probably have the biggest amount of posts, some people do it to increase their post count, but a few do it for fun. But I would disable post counts for off-topic so they won't post in there that much and focus on the other area of your forum.
I moderate a forum which has a nonsense section containing roughly half the full amount of posts across the board. However, as they don't add to the users' post count and its very existence means people don't spam other topics, the management team really don't mind.
I apply the noindex meta tag to my general discussion forum. I realize this costs me some visitors but at the same time it makes my search results much more targeted. I wouldn't recommend doing this if you don't have enough posts in your main categories yet, you need all the visitors you can muster at first.
No it's not more popular, but a lot of post exchangers head over to the off-topic section which is annoying. We have a few regular members that keep are main forums more active, and I work extra hard to make sure those forums are constantly getting new discussions.