We live in a dusty hot region of Canada (I know, hard to believe but true) Summer temps hit around 100*F or more and humidity is 0. I couldn't imagine off-roading without the windows closed and A/C on. Well actually, I don't have to imagine it. Prior to owning the Xterra, we owned a Jeep and thought it would be super awesome to go off-roading in the dead of summer without the top on. Made that mistake once, spent two hours cleaning dust from every nook and cranny of the Jeep and ourselves, and never ran topless in the summer dust again.
the only time I ever had vehicle overheating issues with the A/C on was pulling my travel trailer up and over the passes around Yakima in the middle of an August day a few years ago. We ended up running the heat at full blast with the windows down to keep the X from over-heating. After that trip a lot of maintenance and love was given to the cooling system and we never had the problem again.