When learning to fly in the RAF the message we were always given was "Dress to Survive".
Jeans and a T-Shirt are no good if you happen to have to force land on say, the top of the Yorkshire Moors/Pennines. Even in summer you won't last long and rescue could take longer than you think.
The same goes even for even a short hop over water, even just crossing the Solent.
Again in the RAF we were always taught the benefit of multiple layers of natural fibre clothing was better than one thick one against burns.
Personally, I just don't feel comfortable flying with more bare skin exposed than I have to and prefer to wear long sleeves and long trousers at all times.
MB