The only way we would see a currency being used is if...
1. A large number of tribes in an area agree to use it, or spread through the world there are powerful/large tribes that agree to and use it.
2. It is pegged to an item, or in simpler terms, there are ways to convert that currency unto an item in towns that use the currency. For example, as blackice said, perhaps nails. Every 10 Drokian BottleCaps or whatever are worth 1 nail.

You will never see a more modern style of currency because no one is going to have enough faith in it to accept it as a medium for trade.