As I understand it (American here), it's not only about preventing member countries from leaving the EU. It's also about stifling separatism within those countr

As I understand it (American here), it's not only about preventing member countries from leaving the EU. It's also about stifling separatism within those countries. If Britain can leave the EU without painful trade restrictions, then regions like Catalonia may find less convincing the argument that they must remain part of Spain in order to enjoy the benefits of the EU.