In a blog post on The Test Eye, Martin Jansson has some things to say about the dangers of The Indispensable Worker. The post is worth reading. I commented there, and do a somewhat better job of it here:

Your point about indispensability is well-taken. In workshops that I've attended, Jerry Weinberg has often pointed out the urgency of getting rid of the problem of indispensability. If someone appears to be indispensable, it's a great risk to the organization; it either has become or will become severely maladapted to existence without that person. This isn't to say that people won't be missed when they go; everyone carries tacit knowledge and experience that no one else has. But whether someone will be missed or not, their departure shouldn't destroy the organization. An "indispensable" person will disappear eventually, one way or another. So if you see the problem, get started on addressing it now.

There was a point with which I disagree, though—at least with the emphasis:

As a co-worker you avoid these traps by requiring documentation enough for someone else to perform the task or that you have at least a backup for the critical tasks.

I'd put that in the opposite order. If you've got a backup (in the form of a person who can do that task), then you might not need documentation at all to get the job done; and you might have a better way of performing the tasks that the job requires in the high-pressure moments. This is why commercial airlines tend to have a captain and a first officer in the cockpit, rather than a pilot and a book on how to fly an aircraft.

Note also that there are several relatively high-pressure moments on every flight. Just because something could be done by a single person doesn't mean that it's automatically better policy to have only one person doing it. On the contrary, in most cases.