One valid reason for trading currency futures, instead of cash FX, highlighted in the Refco case:
http://www.cme.com/about/press/cn/05-138CMERefcoStatement16261.html
CMEâs clearing member, Refco, LLC, is a subsidiary of Refco, Inc., but is a separate company with its own accounts, assets and customers. These accounts, assets and customers are protected through a comprehensive federal statutory and regulatory regime and other financial safeguards and risk management protections provided by the CME Clearing House. All customer funds are required to be segregated from firm assets, held in specially identified accounts and are not subject to any creditors' claims against the firm.
http://www.cme.com/about/press/cn/05-138CMERefcoStatement16261.html
CMEâs clearing member, Refco, LLC, is a subsidiary of Refco, Inc., but is a separate company with its own accounts, assets and customers. These accounts, assets and customers are protected through a comprehensive federal statutory and regulatory regime and other financial safeguards and risk management protections provided by the CME Clearing House. All customer funds are required to be segregated from firm assets, held in specially identified accounts and are not subject to any creditors' claims against the firm.