When China’s president, Hu Jintao, visits here next week, the exchange rate between Chinese and American currency will inevitably become a big topic of conversation. China has been holding down the value of its currency, the renminbi, for years, making Chinese exports to the United States cheaper and American exports to China more expensive. The renminbi’s recent rise has been too modest to change the situation, and Mr. Hu’s state visit is sure to highlight the real tensions between the countries. Yet the focus on the currency has nonetheless become excessive. The truth is that the exchange rate is not...