I believe that if they implemented Nation specialization it would fix the XI Jeuno problem (if it is a problem). If each nation had specific abundant resources, and development skills, then each nation would be able to produce larger amounts of those goods for a cheaper price than the other nations. As for a Jeuno type city being a center of trade, well it should be, that's why they put it there.

If the nation specialization was in place, Jeuno wouldn't have any resources for production. Jeuno would simply be a trade center, but it should have much higher taxes (as a crossroads city would tax in real life). Those high taxes would drive people to sell their goods in the other cities.

If we want to motivate players to visit the different cities, there is no better way than nation specialization. If there is an instant global auction, there is no motivation to visit different areas.

As for availability of items, currently I agree that it would be difficult; however, in the future (with a large population) I don't believe it will be an issue. Players will move the goods over into the different cities, and create a living international trade market. At that point the market will become a very complex, and detailed portion of the game.

In the end these basic economic principles will make FFXIV a much better game for everyone.