The bit about the heater isn't all that accurate - the heater is free to use in terms of fuel. the only way it could use fuel is the extra load on the alternator to spin the blower. This is pretty minimal and the heater and blower are independent. My truck flows a lot of air with the blower off anyway (cowl pressure ftw!). The AC of course adds quite a lot of additional load, but the motor is making heat anyway, the only question is whether you want to dissipate it all though the radiator or though the radiator and heater core
