Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Are there any countries where citizens are free to use any currency?
If I understand correctly, you're actually asking why there isn't a society whose members generally accept/use any currency for transactions, and just like, Google the exchange rate or something.  The answer is because it's exceptionally inconvenient.  Can you imagine having a wallet with 200 pouches for all the different currencies?  Why would you want to deal with exchange rates all the time?  What if the value of a currency changes?  (A single currency at least has the illusion of being stable).  Et cetera.