I can only answer for myself here. Ford did a lot of research when they raced this car, trying to determine just how much power it had to put out to make the lap times they knew they needed to win LeMans. One thing they didn't measure, at least as far as I know, was whether they could sacrifice the power, additional weight and fuel mileage that A/C would require and as a result keep the drivers sharper so they would make fewer mistakes and keep up the fast lap times. I suspect they didn't, since race car drivers are generally (even back then) in pretty good shape and able to drive the car at speed for several hours at a time. But the rest of us are not necessarily in such great shape.
I did my own research on this issue. Well, it wasn't a planned thing but an interesting data point I picked up. I went to Bondurant's driving school one summer when they were at Sears Point. Took my '65 Mustang (no A/C). Most of the other students were Ford employees (back when Ford was sponsoring Bondurant, managers were eligible to go to the school at Ford's expense, which greatly increased the incentive for a Ford employee to become a manager /ubbthreads/images/graemlins/smile.gif ) and drove the school cars (prepared Mustangs). It was pretty warm, and after the lapping sessions most of us were pretty wiped out. One guy - his name was Fritz as I recall - wa not only turning in really good lap times, but didn't seem quite so fatigued as the rest of us. He said to the other Ford guys, "did you turn on the air conditioning in your car? I noticed it had air conditioning, so I drove with it on." /ubbthreads/images/graemlins/shocked.gif
I was living in Monterey at the time, and never felt the need for A/C - you really don't need it there. In fact, when I took my CAV out to the coast in August of 2003, I drove it around without the A/C. But when I drove it around here in Utah or Las Vegas in the summer time, or back in Michigan, you bet I turned it on.