I think the reason some people say that the AC does not decrease their fuel mileage is the standard comparison between a car on the highway with the windows rolled up with the AC "on" vs that same car with the windows rolled down with the AC "off."
Yes, it costs to run the AC compressor, but the aerodynamic hit that results with the windows rolled down costs too. It is often said to be a wash. So why not stay cool.
Of course, a die-hard hypermiler can make the point that one should drive with the windows up and the AC off for maximum fuel economy. That might work for the hardiest of us, but you could be arrested for doing that with children on board. :-)
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