Person B has a code base tested by users from around the world

How do you know this? How do you know you're not the first person to ever run it? How do you know the author hasn't completely ignored all patches sent his way? Just because the code exists doesn't mean it has been thoroughly tested.

By contributing back to the code base he insures that his code changes will get also get that testing.

What if management won't let him release the source? What if the project is not open source?

Such an activity may be interesting for the programmer, but it is pure folly from a business perspective.

Is allowing your employees to spend time improving their skills "pure folly?" I think not.

When using existing code you have to take into account licensing, quality, and maintenance considerations. Same as if you wrote the code yourself. The same rules apply. Blindly trusting a piece of software because the source is available is very dangerous.