The problem with using US currency as an intermediary between two other foreign currencies is that the transaction is subject to US law. I can recall a case between two european countries where the goods being paid for happened to be cuban in origin, thus the entire transfer was seized due to the cuban embargo. Between two european banks. That's just silly.

Edit - I guess US interference has happened a number of times, my story included: http://en.wikipedia.org/wiki/Society_for_Worldwide_Interbank...