National borders serve the same function for planet Earth that fireproof doors do for buildings and watertight doors do for ships. They contain an accident so that the entire structure is not destroyed.
Fools who find opening fireproof or watertight doors inconvenient and jam them open will marvel at their cleverness at how fast people, cargo, and money can move from place to place. But the first time there is a fire, or the ship hits an ice berg, of the greed to those in control of the money gets out of hand, disaster results. Instead of losing a single room, the entire building burns down. Instead of a flooded hold and damaged cargo, the entire ship sinks. And instead of a bankrupt nation, you have a bankrupted Earth.
One world economy and one world government sounds like a nice idea. But there are two things wrong with it. The first is easy to explain; what happens when (not if) that one world government falls into the hands of the next Stalin or Hitler (or Bush)? The second reason is a bit more abstract. As long as their are multiple nations, there will be continuing experiments in new ways to organize and operate a society. But as soon as there is a single global government, all social evolution will stop, as a one world government will work only to perpetuate and prolong its owen reign, secure in its self delusion that it represents the peak of civilization.