I've heard interesting things about the inter-connectivity aspect. You can do things like provide resources and industry goods to friends friend. It can also have a DEVASTATING effect if you depend to much on a player that unexpectedly pulls out.

Mike Fahey from Kotaku wrote a piece where he and Stephen Tolito had neighboring cities. Stephen supplied police and water utilities will Mike got his city started. All was well until mike pulled out to start a new city and not realizing that Stephen was heavily dependent on Mike's coal plant. When Mike pulled out, the plant closed and Stephen 's city took a hit. Pretty interesting stuff.