I don't think I agree. It's great for individuals and countries to be friendly, but I see two problems with a one-world-government system:
1. It concentrates an extreme quantity of power in the hands of a few people. Power has a potential of corrupting those who hold it, and I believe that potential increases on an exponential (or at least an algebraic) scale, not a linear scale.
2. If an evil personage gains power (think 20th century dictators), a single world country would suffer significantly. But having countries separated provides a limit and a check on the potential fallout of that dictator's actions.