I have small bantams. I use a SweeterHeater plugged into a Thermocube so it turns on and off according to temperature. It's infrared, and attached to the back wall of my coop, facing the roost. A wireless device ($12.00) lets me monitor the function from inside the house. Ventilation near the roof keeps moisture down. I have a large cage attached to the coop that I open a door to during the day. That area is unheated except for what little sun it gets. Sheltered by the house on the west side, fence on the north and south, and I put plexiglass (these are pets) and choroplast on the sides and top for North wind protection, and rain. Insulation board on the bottom, under a rubber mat with sand & shavings on top. The "roof" and top rear of this enclosure is vented.
Water bowl is also connected to a thermocube.
Maybe they would survive just fine without the heat, but survival and comfort are two very different things. My girls will go to GREAT lengths not to step in snow when I let them out into the larger pen on nicer days. They put a foot down, jerk it up, try again, and then fly over the snow to a bare patch. Our weather can get bitter, with wide swings in temperature. I'd rather keep the coop just above freezing, and not have to worry about sleeping through a sudden bad temperature dip; the Thermocube is on duty!