The VOV theoretically optimizes the pressure of the system and allows for a variable system flow while the car is idling or driving on the highway. While it has an effect on the ultimate temperature output of the system, it is measureable also in capacity, not in temperature.

However, even with an ACC system, if you select 72 degrees, the system might have to push maximum output at times to keep it at 72. Those are the times when the VOV theoretically allows lower evaporator temperatures, which is a plus in hot, humid climates. Obviously that translates to lower vent temps, and quicker temperature change in the cabin. It also puts less strain on the A/C system. Even with an ACC-equipped vehicle, the theoretical benefits of the VOV apply. However whether or not they work in every application, is still kind of up in the air. Some people swear by them and some people don't like them. I don't know exactly what to think, except that the 900 series Volvos have a decent A/C system without it. If temps are extreme it might have more benefit.
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chris herbst, near chicago