I think more people need to read stress signs coming from their birds. Your birds are challenged by weather just like homeless people in the US. The homeless people can survive but take time to talk to them about how being outside in cold feels. Then take into account the life expectancy of such folks. With the typical backyard flock where birds also valued as pets; longevity is often of greater interest than for folks solely interested in nutritional benefits provided by the chickens. Therefore, when applied wisely, supplemental heat for birds otherwise exposed to extreme cold and sometimes even just cold can be a benefit.