With our lot (DHL/EAT in Brussels) uniforms are, hardly surprising, mandatory too, for the reasons stated above - professionalism and identification. However, we don't carry pax and once the door is closed and all that. We do have the occasional 6-hour flights, quite often to rather hot places. Since I moved over to the left hand side, I have been known to get out of the uniform and into shorts and t-shirt while the ship is being loaded. When I was a mere Flap Operator it was follow the lead so to speak, and we do have (like any other operator) a few grumpy old men who stick to the book in all it's petty details (like wearing a polyester shirt in 35C and 95% humidity).
When the company bought the fleet of 757-200SF, I brought one ship up from Tel-Aviv and one over from Wichita; both flights were done in civvies. Catering out of Wichita was quite good, and we did manage to go non-stop direct to East Midlands in just shy of 8 hours.
Oh, we smoke onboard too - how's that! Usually not in the cockpit, out of deferrence to non-smoking crews who'll occupy the ship when we're done with it, but there is a cosy little area between cockpit and cargodeck that's just made for smoking - even the ashtray is still installed.