The folks who just love hammering and teach themselves in their spare time, may well come across an opportunity to hammer professionally.

In which case, nobody will care how they got their skills, whether it was on the job or not. Just that they are really good at hammering and can prove it.

You missed my point. I don't disagree that as long as you're good at hammering, it doesn't matter if you learned it by hammering on the job or because you love hammering so much that you kept doing it by yourself. My point was that some people don't have hammering as a goal. Some people's goal is to make money, and some times that's best achieved by hammering, some times that's best achieved by sawing. But the people who hammer because they like it will try to hammer even when sawing would be more effective.