Indispensable employees are a sign of organizational failure. They indicate a weak point in the organization -- if that person gets hit by a bus, or walks off the job, or is on vacation during a crisis, we're all in trouble.
Easy to say in theory, but in reality, you can't build a great organization that's exclusively composed of average performers. There will always be people who contribute far more than the rest, whether management appreciates them or not. (And in my experience, management is usually very well aware of who those people are.)