Hi Jordan
I also have a S70 (with a parking heater).
Parking heaters and block heaters have been around since the sixties and they make perfectly good sense. Even more so today with cathalytic converters! The point is not that they heat the coupe but the engine. A coldstart is very stressful to an engine. The oil is cold and at the bottom of the crankhouse, so there is very little lubrication at the first revs. The start engine have to work very hard to get the engine going since the oil is cold and thick. And the cathalytic converter is not working when the engine is cold. A cold engine is very polluting and consumes much more fuel. A common reccomendation is that you use the block heater down to +10 degree celcius
To sum up - a block heater decreases the wear tear on your engine significantly, it reduces your fuel consumption and the environmental impact of your car. It saves money AND the environment!
A parking heater is a different story since it uses the cars own fuel. It does all the things a block heater does but at the cost of burning fuel. My guess is that is about 1 to 1 with an efficient parking heater - The fuel it costs for the parking heater to warm up the engine is about the same amount you save on avoiding a cold start. So a parking heater is more of a convenience equipment, like an AC. I park on the street so I dont have access to a power outlet for a block heater. But I still enjoy a warm car on a cold morning, just as I enjoy an AC-cool car on a hot day.
/Patrik 1999 S70 Fully Loaded ;-)