The precedent is that no one thread can stay level-headed for long: all of them develop in to flame wars if given long enough (which is usually in the 70-80 seconds range).
Nowadays we just kill them at the root. They WILL erupt into flame wars, and instead of generating extra "work" for the moderators and hurting people's feelings by pruning irrelevant or otherwise needless posts, we close them before they become problematic.
That about do it, ace?