Hi,
This is just a thought. Although Harbin is near Russia. We must not only looked at own defences (which is China). We will also looked at the air-forces in our opposing country and that is North Korea and Russia in Harbin case.
I don't think there is a lot of aircraft positioned that far north and so there is no need for a vast air defence network in those area. The main battles will be ground battles in those area thus I believe that China is positioning her ground troops mainly in those area.
As for air-defence network, if you actually look at the distribution in China, they are mainly in the political center which is Beijing, and also in cities and regions near the sea. This is because their main enemy that could attack them would come in from those area. And these enemies are the Taiwanese adn the US or western countries.
As for the Indians, I do not think they are a major thread to China yet. And the Indians are not known for their air-force. Thus the main thread is ground base. Thus China had vast army and ground forces near those area.
In short, when we look at where and why those air defences network are being set up, we do not rank the threats of individual nations to China, but instead we rank the type of threats to China. And studies into the theory behind individual nations' defence and attack policies, their standpoints and their military distributions would give us some idea into where we would want our air defence, air force, ground forces and even our missiles troops.
The threats to China by any country is the same in the view of the military, however the 'enemy's' distribution of arm forces and their capabilities (for the US, and western countries - we will be looking at air force and navies, thus the concentration of air defences at coastal areas to defend China from these forces, and ground forces in North Korea and Russia, thus ground based forces in Harbin, Inner Mongolia. Terrorists in Xinjiang and Tibet, thus ground forces and armed police in these areas, etc).
The above is just what I have analysis. Feel free to topple them in whatever way.