Just after 9-11 it was suggested that pilots wear street clothes so that they could blend in with the passengers. The reason was so that they would not be a target for terrorists.

The FAA put a stop to is since they felt that during an emergency the four bars would command respect from the passengers.

Imagine going to work in business casual. I took a maintenance ferry flight once in the 757 wearing a polo and blue jeans. It felt strange.

Skyhigh