best to run your aircon for a few minutes every day, keep it fresh, in the 197 i rarely used it, when i finally did it smelt like something had crawled into the vents and died.
Benefits far outway the negative. Fuel consumption isnt really noticable. doubt its even as high as 0.5 mpg. Driving with the window down will affect your fuel consumption too.
Think of when you wake up on a cold morning waitin on a window demisting, or when theres a hosepipe ban in the summer and your in you car sweating like a pedo in a playground. Not for me.