A one world government and the homogeneity of governance that necessarily follows is a terrible idea for a myriad of reasons. While it seems a laudable goal, this perception is the product of shallow, optimistic analysis. The main reason people think it is a good idea is because it would, in theory, mean that wars were not possible. Obviously, however, this isn't the case. Civil wars and insurgencies have the same results as wars between nations: death and destruction.

In addition, imagine if the world government became malevolent. As it stands, if our nation of residence is becoming oppressive we can flee elsewhere. Under a one world government, however, where could one flee to escape it's totalitarian clutches? Further, what force could oppose a malevolent world government? When Adolf Hitler set his sights on world domination it was only other nations that could stop him. Those within his empire had no such ability and even the French resistance required outside aid. The French resistance also only existed due to the fact that the French army wasn't entirely militarily crushed because they were conquered so quickly by the German Blitzkrieg. If there is a malevolent one world government, there will be no force that can oppose it and emerge victorious.

Further, one must think of the manner in which we have progressed as societies in the past. Different nations try different methods of rule and we can compare these to inform our manner of governance. One might find, for example, that giving one's citizenry greater freedoms results in greater productivity. This is a big reason why nations grant freedoms to it's citizens; in the interests of what is pragmatic, rather than in the interests of morality. Different nations and empires each found different methods of progression at different times and intentionally or unintentionally shared these. For one example, the renaissance is at least partially creditable to the manner in which the Medici ruled Florence, including their patronage of artists, inventors and other polymaths.

Finally, one must think about the ability to be represented by one's government and it's accountability to it's citizens. When one is governed on a smaller scale, one can be greater represented by their government. This is both because one makes up a larger percentage of the voting population and because different issues have different importance to people living in different areas. This is demonstrated by, for example, the cities of the U.S. being more left leaning while the countryside is more right leaning (Source 1). These contrasts are magnified when one considers the political and ideological differences between nations. Moreover, under diversity of governance, one may be able to choose between several available options which government they prefer. It is also an immediately apparent fact that a government is more accountable to it's citizens when it is more local. A multinational government based in Brussels, for example, has less reason to be afraid of unrest in London than unrest in Brussels. Moreover, it is easily demonstrated that at larger numbers accountability and representation is reduced. If one has 100 citizens and one ruler, it merely takes ten outspoken citizens for a problem or concern to be made apparent. If one has one million citizens, however, it takes one hundred thousand protestors to have the same effect.

Sources:

(1) http://www.economist.com/blogs/democracyinamerica/2010/11/political landscape