How do you heat up the cabin in your light single? A heat muff surrounds the engine exhaust that would heat the outside air and then pipe it into the cabin. It’s quite energy efficient since it’s simply using the by-product of running the engine so there’s no extra energy required.
Alternatively, light twins have a combustion heater in the nose of the aircraft. The heater is similar to the ones at your home, which burns gas or fuel to heat up air. Since we’re flying, fuel is a very important resource, so why did they design cabin heat in a way we’d use more fuel and decrease our range and endurance? It’s to provide us with more cabin heat. But it’s not solely because the combustion heater can heat up the air more, which it does, it’s due to the path of the hot air ducts.
When would you encounter conditions where you would like to have cabin heat on? Most likely, it would be when the outside air temperature is <5°C. Recall the environmental lapse rate. If 5°C on the ground, at 3000agl, it’s -1°C. If the twin uses a heat muff, the hot air would need to be directed through the wing then into the cabin. All the extra air duct would cool down the hot air so by the time it reaches the cabin it’ll be tepid at best. I would gladly accept a reduced range and endurance if it means my hands and feet are still warm enough to aviate, navigate, communicate.