For my own birds, I do think there is good reason not to use heat lights. Any time you have a heat light, there is some chance of fire, no matter how well secured you have it. I know of a number of people that have burned their coops and with them, their chickens. I'm sure they thought the lights were well secured as well. Sockets can short out, bad connections can cause heat in the wrong places, rodents can chew through wires, dust-bedding-feathers can land on the lights. Any of those things can cause fires. Even if none of that were true, you mentioned yourself you like the birds to acclimate. We had temperatures of -40 F two winters ago for two nights in a row, with many nights in the -20 to -30 range before that. Suppose my birds were used to heat lights being on at 10 F. Now suppose we had one of the -40 F nights and my light burned out. Without heat, my birds are well acclimated to cold. If the heat light had taken the temperature up even 15 degrees, and it burned out, the temp just went from 25 to -40 in a very short time. I can't believe that is good for any living creature. It has nothing to do with how cold they can stand to get without dying. I have raised them both ways, and my birds are healthier, happier, and more active in very dry, very open coops than they are in more tightly closed, warmer coops. I'm not waiting until my birds get frostbite to give them heat. They don't get frostbite now that I keep the coop bone dry and draft free. My birds in the past in an insulated tighter coop did get frostbite.