It is a currency but what crypto currency (or, in fact, any alternate currency) lacks against 'traditional' currency is a floor of value. For instance, no matter how much collector value an old US Dollar bill holds, it will never drop below 1 USD in value as it's a promissory note from the US Government that it has a value of no lower than 1 USD. Alt-coins are not backed and can be any value, from thousands of dollars per coin to exactly worthless.