I can see a lot of merits in designing a world on this kind of basis... if you're running a pencil and paper game with a known group once a week.

Short version of theme that it sounds like the author is implying (with plenty of extrapolation on my part, of course):
About 1400 Europe or so. Most people still live as peasantry. You have your normal governmental types, artisians and military... but one difference. In this world the power of magic is being discovered. In this game magic is new, just being discovered, and more than likely has some science based reason why it works (even if we don't quite know it yet, I envision what he has in mind for magic is like the 'magic' of Fullmetal Alchemist, that is, a form of advanced alchemy that looks like what we today view as magic). Magic is likely something that takes years of study to be able to do well, and while some military applications are being used now (perhaps using water to create fog to confuse enemies), it's mostly a science where most of the effort is still spent trying to enhance the food supply and keep living. That is, perhaps mages are viewed more like tradesmen than anything special, and they might even have a guild.

Then find a couple reasons 'adventure' type PCs could exist, even close off mages to initial application, and off you go.

This doesn't even go into the concept of fantasy clerics, but... wow, this actually sounds like a theme I'd enjoy GMing.