You have a good point. I think it would be more of a body style thing determining how much resistance you create with the windows open. I don't believe it would rob as much power as the AC would.

The A/C compressor draws power from the engine via belt. Engaging the A/C induces an additional parasitic loss on the engine leaving you with a choice; generate more power to compensate (using more gas) or slowing down. I generally take a 10% hit to my MPG when I use the A/C in my '94 Subi.

Open windows also generate drag, but whether or not the resulting inefficiency is more or less than that from using the A/C depends on the car and the speed. Then again, at high speed I get a nice breeze through my vents as air is forced into them harder so I don't NEED to crack a window at interstate speeds. In fact, doing so may cause handling issues due to turbulence.

I heard on one of the news shows, that air conditioning on in the car uses less gas than having the windows open.