An NPA connection also may be the tendancy for N types to have low dopamine genetics. This is a condition associated with propencity to seek substances. Although as NPA theory points out any NPA type can be prone to substance abuse perhaps because many genes affect the dopamine activity in the brain.
As for N types and not being hurt by others' opinion of them, that is indeed a huge plus. When one considers the virtue level of humans, one should make ones own judgement about his own worth while at the same time never losing sight of the dangers posed by others.