Nothing can beat natural light and the feeling of a comfortable setting in which the photographer vanishes out of your immediate view.
As I know by own experience, nothing is more scary than being in the spotlight. Surrounded by flashlights and forced to smile.
I feel that this unpleasant feeling comes across in set up pictures and if possible, I am trying to avoid any set up and shoot when my "models" don't necessarily notice it.
This authenticity that pictures like these represent is the foundation to my understanding of a good portrait.