For interest there was a film at the Survival School at Mountbatten (then St Mawgan) showing the fire retardent properties of the nomex suit and what happened in fires etc - after seeing that I never wore anything else for flying, with the appropriate kit underneath as well!
people moaning about being too hot in cockpits in the summer and wanting to wear T-shirts forget how vital the cotton layer underneath the nomex suit is to prevent burns.
I still wear the roll-neck and long underwear every flight - you only have to see one victim of burns for it to convince you that a little discomfort is much better than a lot of pain and scarring.