Rel's commentary should be in the next DMG
I concur completely.
This is something I wish more players, and more DMs or GMs of any game understood, the obvious advantages of - establishing, maintaining, and plying long term networks and networking contacts.
Informant networks, financial and financing networks, expertise networks, contact networks, political networks, religious networks, social networks, professional networks, and so forth and so on.
By establishing and maintaining viable and valuable networks and contacts wherever the characters go the players build up a market of expertise and ability that compliments their own, and can assist them whenever they really need it (and the reverse is true of course, networks are never one-way streets).
And by encouraging networks GMs build up a much, much more believable not to mention much more interesting world, than just continually laying out a never ending multitude of single term contacts whose only real value is to work one event, action, or incident.
It's just like in real life. It's far more practical and valuable to have a personal cadre and cache of human contacts and expertise to draw upon than to try and establish a new set of contacts on the fly to help resolve every problem you encounter.
Yes, occasionally or maybe even often you want to expand your personal networks, of course that's true, you want to replace extinguished or unreliable contacts, and want to add to the capabilities of your already existing networks, but instead of looking at every contact as a single-use function, you look at others as a long-term resource that can be exercised multiple times for problem solving. And you don't have to like or agree with every contact in your network, especially in networks like informant based or information based networks. You don't have to like them, they just have to be useful and trustworthy. (And if they are not then replace them with someone who is. Better fewer and more reliable contacts, than a multitude of untrustworthy ones.)
And like Rel said cities are an especially useful place to establish such networks.
That was a great series of observations on networking.