The idea of one world and one voice is a great idea. Such a system would give everyone in the world an equal voice. Regulations on occupational standards and environmental impact would be the same across the globe. These two standards alone would improve economical issues across the world that are points of contention between trading nations. Unfortunately, such a system will never exist.
We are individual countries that can't get along as it is. Why would we get along if we were one. We have to many differences to be able to be one globalized society. Nationalism is better we keep our individual identifications of the different countries we are recognized as each one.
Everyone has to be from somewhere. The people would still see themselves as where they are from. It would also make powerful men and women that much more powerful;l. To much power is just bad for people. They think it is good but all they do is create a more chaotic world. Which, in the end just makes their job that much harder.
One globalized society would just give a small group of people even more power, it wouldn't make us as a people more free or happier. In a fictional world that human nature wouldn't want to abuse having power over the whole planet, this concept would work, in reality it doesn't. We're not perfect as we are but better than that'd be.
I do not believe that creating one globalized society and dropping nationalism would benefit the world. I think that creating such a society would only benefit the poorer countries and the corrupt. There would be no benefit to modern Western countries like the USA to such an idea and concept.