"Exactly this. most auto AC units have an evaporator sensor to help prevent the coil from freezing. At a higher fan speed, more refrigerant is permitted through the coil to maintain an even discharge temperature. I had one fail recently (under warranty), it takes about 30 minutes at 95F/70RH to freeze a coil to a block of ice if the refrigerant discharge is not regulated. "

They have apparently changed again then. All though the 1980s it was a cap tube/critical charge system. Cheap, compared to the old ones that had a real thermostatically controlled expansion valve and an accumulator.

The ones I saw simply worked on low side pressure. The lower the pressure, the lower the temperature. It also served the purpose of keeping the compressor from sucking in air in case of a low side leak. But of course most leaks are on the high side.

I haven't worked on alot of newer cars, but I do know they are doing alot of things to improve efficiency, like variable valve timing which is pretty expensive to implement. But so were roller lifters at one time. I am not sure how much they're getting out of the direct cylinder injection but if it raises the power to weight ratio that is a plus. Both of these things were unheard of in the 1970s.

It was said the AC takes about 12 HP in a car. People have told me also that the performance is off when they have the air on. Some cars had a switch to temporarily disengage the compressor at wide open throttle. Not sure if they still have that or just use the computer to do it.

Another reason they need some sort of regulation is that most cars run the compressor when you use the defrosters. It allows it to start clearing the window long before the car is warmed up. In the 1980s GMs at least you could hear the compressor kick in and out in the winter as if you are low on Freon, but it is because the temperature is so low. Obviously the heater core goes before the evaporator but if the engine is not warmed that means nothing.