As RPM said we're pretty tight knit around here and keep everyone apprised of any extended absences.

Here at Bifrost just about all the stories are Free Form.

http://www.bifrost-bridge.com/mimir-s-well-f4/starting-your-story-important-information-t71.htm

We tend to shy away from Directed and Controlled stories for specifically the reasons you stated. In our free form scenario's, no one really calls the shots, but occasionally someone might jump into the role of 'game master' to portray villains or steer the story. We keep it relaxed though, and so if something does come up, the other writers can easily work around the posts of the absent writer, preventing the story from dieing.