how about this..
A certain number of loose ball bearings can fit in the race. Each ball has a contact point which carries a load. BAlls are side-by-side, touching eachother. The more balls the better, no?
But loose ball bearings are a pain in tha ass to deal with so many times they use a retainer instead. These balls are not loose. And since the retainer takes up space between the balls, the balls are nowhere near touching each other.. By necessity, there are fewer balls. However, these types of retainer bearings are found virtually everywhere the loose balls could be used. Balls are spaced out and there are fewer load support points.
So, if the bearing races are lubed, all parts are smooth and properly adjusted, i don't see how missing one ball will make a difference in the real world.
I'm not trying to justify being lazy and so not reinstalling the found ball bearing.. i'm just logically examining the value of a single missing ball.