Clay Shirky argues that cities are an example of the vigor of decentralization, not (as some argue in the post-9.11 world) a vulnerable centralized point-of-failure.

New York City happened not because the Bureau of Centralized Cities decreed that New York City should be the largest. Indeed, at the founding of the United States, either Philadelphia or Boston would have seemed liklier candidates for that sort of pre-eminence. New York is big because over time more people came than left, because millions of uncoordinated actors decided independently to move to New York. The population is not a single variable, it is the sum of these countless distrbuted decisions.

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