Licensing isn't about magically ensuring that the practitioner won't make mistakes; it's about holding the practitioner accountable for his mistakes, which in theory gives him more incentive to be more careful -- or to change his practice's workflows and systems so as to be better able to detect and correct mistakes.

In fact, I would argue that the "throwing more people at the problem" phenomenon in healthcare is an example of that very thing. Do you think they'd keep staffing levels equally high without licensing? 'Cause I sure don't.