Hm, wouldn't mean that the 'delta' idea does not work after all ?
The reason you did not notice it was that all strings you received from the roots and the translated string keys had no difference.
I'd say a bit more than convenience. It's a way for the 'language guys' to provide the final strings, maybe even in some Excel file. (Almost all grenades for example have different names from the ones in the translated string key files.)
Then have some script run over the provided data to build the different language XML files.
