You'd be surprised to see how often this occurs. Sometimes, for piloted projects, especially during the prototyping phase, a single point of failure—or, as pest

You'd be surprised to see how often this occurs. Sometimes, for piloted projects, especially during the prototyping phase, a single point of failure—or, as pestatije pointed out, "bus factor" — sometimes inevitable. So you either run the risk of that person being no longer available, or, even during the initial stages (which can be expensive in human resources), partner up, right from the beginning.