Like I said, documents only have value if they are accepted as valid. Currency, local dollars or otherwise is no exception. Currency is pretty much the same as postage stamps or passports or marriage certificates or other contracts. (Notice that all the above mentioned are contract agreements between at least two parties often with a third party regulating validity)

Trouble with SixNation issuing passports is they are not always honoured as valid. Sweden and Switzerland accepted their validity. UK has not.