Human population grows rapidly, while areas suitable to establish settlements remains constant. It forces mankind to build multistory hive-cities with extremely high population density.
In peace time hive cities grants some advantages to its population and local control units. It's easier to provide food, water and power supplies in hive-city than in small settlement. Hive-cities allow to create well-developed infrastructure and jobs in plenty. So, big city is a complicated organism, and as other complicated organisms, it's vulnerable.
Modern weaponry is deadly instrument of imperialists' will. Imperialistic hydra can easily unleash it to fulfil her predatory desires. In this case big cities will be primary targets. Exrtermination of main cities will paralyze the country and her will to resistance. You can say, that menace of nuclear strike, massive bombardment or chemical attack is too... ephemeral. But you can face even worse scenario - disturbance, destabilization of situation, and mutiny at the end. Hive-city population can decimate itself in bloodshed.
As you can see, hive-city is not a bad thing. Especially if you have some sort of escape rout. ^^
