My original point was a currency of some country would be useless to another country because each of their governments wont accept another country's currency to pay as tax. If the United States government started to accept BTC for tax payments try to imagine what effect that would bring to it.
Well, I actually thought you were talking about Bitcoin, after all
Another thing to discuss is its also good if the government or whoever starts accepting BTC as a currency. It would mean we could also pay our taxes with it. One of fiat's important function is to pay what we owe the government with it. Its one property that makes fiat valuable
In fact, it was my point exactly that a foreign currency would be mostly useless in any country apart from its home one, and I told you that in this respect specifically Bitcoin won't be any different from any other foreign currency out there (unless the latter is accepted as legal tender, of course). You seem to have already forgotten who has been telling what