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I sure wouldn't call is waste, I like to stay warm, but during a long car ride my girlfriend asked me the same question, does it use more gas? I had some time to think about it... there are two other very small power consumers to consider:
A stretch, but the fan you use to get that heat into the cabin takes electrical power, requiring the alternator to work a little harder.
A real stretch, the waterpump works harder to push the coolant through another heat exchanger, increasing the surface area in which the coolant must flow over.
Again, these are both big stretches, and small consumers of ultimately slightly more fuel when you run your car's heater.
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