I can't even remotely explain how important I feel the "hit by a bus and now we don't have any who knows [x]" behavior in companies is (or rather, the lack of said behavior). I think it's just a common behavior when you have a single person work from start to finish on a given section of a software application: you have the guy who wrote the interface, the girl who wrote the server application, the guy who built the installer. Ask one of them to take over for someone else during a vacation period,...