Well, that would work... but it has some disadvantages:

1. It means a new node for every announcement, forever.
2. It would (maybe) make the RSS feed harder to implement — or at least more costly, DB-wise, having to spider the thread.
3. New items would appear at the bottom of the page, at least for those users who have their settings that way.
4. It would be hard and/or messy to roll off older announcements as they become stale. Wiki nodes make this trivial.