Barry - it’s not a trademark thing. People have been talking about it since before you and I were born, both here and in Europe. What matters is the metabolism of the vine and the grapes. In Mendoza, where the lowest elevation vineyards are higher than anywhere else in the world, the question is slightly different. There, the atmosphere is thinner, the sun and particularly the UV rays are stronger, and you want cooler regions overall because you have to manage the sun load.
In places like Napa, where some vineyards are very low and others are relatively higher, you have a different spectrum of issues you can manage. The point of taking advantage of diurnal swings, if you get them and need them, is to help regulate the metabolism of the grapes. If you’re in a sweet spot, you might not want significant temperature differences. But if you have warmer temps than you might like during the day, you look for a place where they’ll be cooler at night. The vine wants to keep working and if the temperature is comfortable for the vine, the berries will continue working around the clock. We on the other hand, have different expectations than the vine does, so we may not want as much sugar, we may not want the acidity mix to change, etc.
That’s what terroir is all about. It’s never one size fits all so that around the world mountain fruit is better than valley fruit.