This sounds like an assignment.

Mike, what is your opinion? If you're going to start a discussion, at least tell us your ideas first.

Yes and no. If you only study the major cities then no.
The bigger the city, the bigger the tourism and more immigrants. 
Society is made up of a group of people that share the similar culture, values, folkwways, mores, and taboos. 
Places with people from other societies for immigration or tourist reasons could throw you off.
It's best to study a balance of big and small cities. To get an idea of rural and urban life within the society. 
Plus, some societies only assist their biggest city to make their country look better than it really is. 

Umm...what's the most important characteristics of a society?  does it mean to reveal the reality of the country?  

Define the "most important" characteristics of a society first.

haha, this is GRE issue topic, and in my opinion, not only learn form its major cities, but also small town and rural areas. For example, there are many big cities become globalization in my country like Beijing, Shanghai, GuangZhou and Shenzhen and lost its characteristics. People should go to small towns and rural areas to realize one countrie's characteristics