You people keep repeating that. I have yet to hear any reasoning supporting it. Is it because your grandfather told you so?
I gave you some reasoning, the best that I could do, which was based on experience.
Take an old cup and cone bottom bracket with caged bearings, pack it with fresh grease, and spin it. Listen to it as you spin the cranks. Feel it as you spin the cranks. Spin the cranks and see how long the cranks spin after you let go. Then take it apart and repack it with loose balls and fresh grease, and repeat the test. Or just do it on another bike, with a similar bottom bracket, if you've got one. Then get back to us and see what conclusion you reach. The difference with headsets, as already agreed upon, is even more noticeable.
I've never heard anyone say that they repacked a (fill in whatever part here) with retainers on a bicycle, and then say it works smoother and quieter than it did with loose balls. But I've heard the opposite quite a lot. Why do you think that's so? And by the way, my grandfathers were great guys, but neither one of them worked on bikes as far as I know, and they never advised me on them. Nope, I just used my eyes, ears, and hands to reach the conclusion I have regarding ball bearing assemblies on bicycles-