Aha! Now I know why I saw a female BA cabin crew member wearing a hat the other day at T5. I was wondering if she chose to wear a retro-uniform for a special event or something. I thought the hat looked rather nice.

But I can see it being annoying. It crushes one's hairdo, it's another thing to leave on the crew bus, and it's another thing to keep clean etc. And put on a wrong way, it can look daft. Not exactly the most practical thing to do, especially considering that BA is trying to cut costs - uniform hats cost money.