Never used to wear one when flying aeros in the UK. In France you have to wear one by law, and I find it quite reassuring.
To re-affirm what Croqueteer said, things which seem easy sometimes aren't when you don't know what's happening to the aeroplane. I learnt inverted spin recoveries to the point where I could do them safely (didn't like them), but when I entered an incipient inverted spin in a Pitts S2A, it was a good thing there was an instructor in front to recover. It was very disorienting.