I agree, especially since i'm also guilty of going with the flow of the discussion a lot of times. A secondary thread would keep the update threads clean without muffling people who want to discuss secondary issues.
There's only one problem in this idea, it takes more than one moderator to enforce something like this. For example, one person would have to be constantly monitoring the update thread and when secondary subjects arise he would have to move the offending posts to the secondary thread. As long as people where not offensive to each other there would be no need for bans or warnings, just moving the posts to the secondary thread.
Of course, that would make the secondary thread a kind of raw dump of pruned posts and it hinders the flow of conversation somewhat when posts appear out of nowhere, but we can't have everything. In fact, just because of posts disappearing from one place and appearing in another, after a while people who wanted to discuss peripheral issues would learn to go to the secondary thread right away and structure the discussion as usual, instead of having to locate their moved posts every time, or if it had a lot of concurrent themes they would start a new thread of their own for a specific issue.
I still like the idea however. Instead of telling us "you can't talk about this here", it would be like telling us "you can talk about anything you want in that one room, but you sift through the rabble yourself"
