I buy loose bearing balls for about 2 cents each. So I always replace them. The time taken to tear down, clean and regrease a hub or bottom bracket, spending under 50 cents on replacement bearings is trivial.

I’ve seen a wide variety of things when opening old hubs and bottom brackets. Not knowing the bikes histories I don’t know the causes.

+100 caged bearings tend to fair the worst. When the cage ass-plodes it tends to do other damage.