Most areas are "good enough" to make a self-sufficient city. I have yet to rely on another city for stability.
That being said, cooperation is important for a specialized city to reach maximum efficiency. Once you've established the basics, the limited city area leaves you with little room to expand buildings important to your specialization.
For example, let's say you want to manufacture electronics. To do this in a single city you need:
- Educated sims, meaning universities, colleges, high schools, elementary schools, and libraries.
- High-class residential for workers, meaning lots of parks and services.
- Consumer electronics factory.
- Trade buildings for trading globally, shipping freight, and improving your trade.
- Boats and trains to help ship freight.
- And finally, TONS of high-tech industrial.
By the time you cram in everything up to the high-tech industrial ("everything" takes lots of space, trust me), you're left with little space for expanding your factories. But your factories are your primary source of income in a manufacturing city! This will heavily impact your freight and electronics output so you won't see the godly profit you should be making.
If you can get your basics like power, water, police, fire, healthcare, and even your educated/high-class sims from somewhere else (this is possible with another university city or the group project Arcology), then you save all the space dedicated to those basics, meaning you can make more manufacturing and make a larger profit.
Something to realize is that this is not only good for the manufacturing city. The manufacturing city greatly benefits from recycling, so you can save every other city in your region the trouble of dealing with recycling and help them reduce their garbage. The manufacturing city also specializes in high-tech industry, so you can provide freight to other cities so they don't pollute with their own inferior industry. Combined this makes other cities very clean, which lets them focus on strengths like tourism and education - you get the idea :)
You might be wondering, "how can a city providing the basics make godly profits?" On top of the money that cities have to provide you for your basics, you can pair the basics with what they support well. A city with a strong police force can handle the horrendous amount of crime from casinos. Casinos spill crime to other cities, meaning other cities will want more of your police, meaning you expand your police force and can support even more casinos. Circular profit enabling - yes please! ... just be careful the crime doesn't get out of control. Learning which basics and specializations pair well together is what will make your regions successful. Some pairs I've thought of:
- Industrial cities need strong fire-fighting to handle hazmat fires.
- Education cities have low power and water requirements and can research and provide clean power and water.
- Cities with lots of garbage and low-tech industry need strong medicare for sickness.
- Trading cities need recycling to create plastics, metals, and alloys.
- Commercial, tourist, and casino cities need strong police forces to handle crime (as I've already said).
- Get basics from neighbours.
- Build tons of specialized stuff.