Chemical Air conditioning? Maybe you are on about the refrigerant in the AC system? A/C Just uses a special substance with a weird phase change temperature to move heat around like a heat pump. The refigurent is in a (theoretically) closed system, So you will never have chemicals used up or put in the air unless your AC system has a leak.
The air con would allow you to use recirculation without fogging up the windscreen. Using the A/C WIll help reduce the condensation problems in the MG as it dehumidifies the air (that why you get a dripping from car in summer, its from the AC extracting moisture from the air)
When using auto mode, the A/C allows for a more consistent temperature.