Training in the Canadian winter, you learn a few things... Your sweat will condensate and freeze inside the last layer of material. If that's a sleeping bag, those ice cristals can melt and make it wet and miserable. If it's your mylar, you can just shake it off.

I agree the mylar can make you sweat more & make your clothes / sleeping bag more humid. But due to their weight & bulk they are invaluable in many situations.