I was pleasantly surprised to drop some late paperwork off to the crew of a B74F (in another life), and witness them resplendent in Jeans and Polo Shirts for their long-haul leg.
Guess what - the plane flew just the same!
The uniform is an image thing - makes the public feel good. Thats about it really.
Hey - it was a total rip off of a Ships Crew uniform in the first place anyway (by Juan Trippe)
If I am doing a double shift (Morning / Night) (and then probably on overtime) the safest uniform for you and your aircraft is the one that I feel comfortable in.