Hrm. I'm not sure it really does. Although you make a good point.
But I like the ninja example, I'm going to run with it a bit. Below is speculative to make a point, not historical.
I would say that the ninja's required skill is stealth. The ninja must have practices stealth to an alarming degree.
After that, I imagine that the ninja's support skill levels might vary widely. One might be amazing with poisons, another might be able to throw a star with crazy accuracy. But others might just have adequate skills in these areas, and still get away with being ninja.
This is the approach that I advocate. Have a primary kick that you have practiced 10,000 times, hopefully one that's very effective for you. Then, maintain a secondary skill set that supports your kick... a few good blocks, punches, footwork. Finally, try to have a superficial knowledge of as much as you can.
If you do this haphazardly, it generally ends up less effective. But if you strategically maintain your secondary skills to balance your primary skill, I find this is more effective than really putting all your effort into one thing.
But Bruce Lee and Seb's point stands. I find that the cornerstone skill is usually present, and serves as a foundation and reference point for where you develop your other skills. Because a real cornerstone skill takes on the order of a decade to really develop. But it is often exponentially more effective, and a borderline requirement for superhuman achievement.