I've briefly been trying to think of why you might have road access and a helipad on top of a mesa. Two that spring to mind are either a) a large house/mansion (built on top of the mesa for the views) for a guy/girl with a garage of cars and a helicopter for his commute into town for work. Or b) some sort of helicopter tour operation (a la the Grand Canyon).
However, your idea of a decommissioned military base with an attached helipad sounds like a much more viable option!
And to answer your other queries:
The rules for the UK and the US are slightly different but if I was to build a helipad, I would rather have it outside the town that within it. The land will be cheaper, you'll have less bother from your neighbours (due to noise etc) and it will be less restrictive for the types of helicopters that can use the facility. (There are different rules for if the helicopter has 1 or 2 engines and if it is a private flight or public transport). But, like I said, the rules and attitudes are different over here. For example, I know that there are helipads in Vegas in places that we wouldn't be allowed helipads by the UK authority.
Wind is obviously a consideration but it would have to be consistently very high (50-60mph+) in order to persuade you to build elsewhere. But other weather considerations might be more important - is the top of the mesa regularly 'in the clouds'? If it was, then yes, I would build the helipad on the desert floor.
Edited to add: My reply has come between a couple of your extra posts. I can't type quick enough to keep up with the questions!
But simply (in response to Gargleblaster's point), yes, the news heli can land wherever it wants (as long as it has got the necessary permissions). The pilot would only HAVE to land at a helipad if he needed fuel. But if it was a decommissioned helipad, it probably wouldn't have any fuel so this wouldn't be a factor.