Why Should the United States Be Concerned If China Stops "Manipulating" Its Currency?

The NYT printed a Reuters article which included the bizarre assertion that the United States would be in some way threatened if China stopped buying US government bonds. The assertion is bizarre because for years many people (included me) complained that China was deliberately keeping down the value of its currency against the dollar in order to support its exports.

Depressing the value of the Chinese currency resulted in the country building up a huge trade surplus with the United Stat...