[Fyron]

A unified world government is _never_ for the greater good. It is for the greater _evil_. The bigger an area a government controls, the more tyrannical it becomes. The fewer external enemies a government has, the more it turns upon it's own people. Absolute power corrupts absolutely, afterall.

Some sci-fi universes have more of a UN type world government than an actual unified one. Babylon 5, prior to Clark creating his dictatorship, had Earth like this. The "UN" body had actual power, but it was still essentially a confederation of semi-autonomous nations.