Semantics?
Hello Eric:
No, air conditioning is not "really" necessary on expedition vehicles - whether you mean cooling or heating. Most dual sport bikes lack cooling and the majority lack heating, for example. As you mention there was a time when cooling wasn't available and an earlier time when heating wasn't available. However, would I want a modern enclosed-cab expedition vehicle without heating? Absolutely not. Would I want one without cooling? Most likely not, unless I knew I'd only be spending time at really high latitudes and or elevations.
I suspect we all appreciate the advantages of heating and probably don't need much discussion about it.
For me the advantages of modern cooling systems far outweigh the disadvantages. Aside from the obvious comfort others have mentioned, I like cooling for the dust control it provides. While I have fond memories of long Baja trips in my 1967 Ford Bronco w/o cooling, I also remember inches of dust over everything.
I guess the main disadvantages of a cooling system on an expedition vehicle are increased complexity, decreased fuel efficiency, and more stress on the vehicle's engine cooling system. There are times when making long, steep off-pavement climbs during summer trips when we watch our engine temperature and turn off the cab cooling to keep engine coolant temperature reasonable. I'm convinced that modern vehicles have the systems well enough in hand that the increased complexity of cooling systems isn't an issue.
Howard