Re-using ball joints?

I've sold numerous brake and suspension parts on this forum, throughout the years, without anyone ever getting back to me, with complaints in reusing upper control arms, that have nice and tight, good ball joints still installed in them.
What problem is there in using something that probably has a few thousand miles to them, that were replaced, before the vehicle got smashed, and turned up in a Pick-N-Pull wrecking yard.
You put used engines, and transmissions, and drive shafts, with the old universal joints still in them, so whats the difference if you put an excellent control arm back in service with, ball joints in them that are in good condition.
Some of you guys out there, make mountains out of mole hills.
You aren't building the space shuttle, or the Martain rover.