The worlds urban population is now about 3.3 billion, and is expected to rise to about 5.3 billion in 2040. The proportion of urban-dwellers is about 45-55%, and there seems to be no reason to be pessimistic about this.
The World Bank in its Wold Development Report argues that this has wast economic advantages. It is also connected to rising standards of living and better efficiency in agriculture and manufacturing. Gradually the standard in the cities and in the countryside will converge as they both become more efficient. This process has taken place before in western cities, and are now progressing to cities in many developing countries.
Cities are created as a consequence of trade. Market towns trade agricultural produce, secondary cities produce and trade industrial goods, big cities design, make and sell everything, including services. Examples are the new big cities in China.
This has been made possible by cheap transport which has made possible a flow of intermediate products and clustering of skills. Consumers flock to the places where assembled products can be bought, together with financial services. People are migrating to economic opportunity!
Countries without big cities may fall outside this development. Urbanization should be encouraged and there should be no pockets of trapped people. Therefore let cities grow by developing markets and improve infrastucture. Advanced cities must spend money on waste and crime to keep up the positive effects.
see link: http://www.worldbank.org/wdr2009