You can't just trade them, but you could find a loop where every two consecutive traders have at least one community in common.

Perhaps I don't really understand the value of not being able to convert them.

The point is to incentivize local spending, not just spending on local goods. Money that circulates within a community stimulates more local activity. Local producers are more likely to undercharge for the value of their goods and services for the sake of community vitality than large, remote businesses that want to squeeze every bit of value out of a transaction.

It's essentially a way to get more work done. It doesn't work if the network is so large that people lose that incentive.

Right, so this is a way to make local economy more efficient that works because people care more about local externalities.

The question is, is there a way to scale this to the whole world by building a global network composed of local networks?