I've been using a similar system in Presence (flyingmac.com) for a few years now. It's great for those rare emergencies and I agree its use should be sparing. I do use it to display release notes though.

If you're using preferences to record when a one-off news item has been displayed, 'ware the guest user, whose preferences are deleted every logout. Someone has just reported that Presence's news window displays every time the guest user logs in, since viewed items are never recorded.