Gold in them thar hills? Pffft.

Gold is used as currency mostly because most new CRPG players knows that gold is valuable and they should collect it. It's simple to have "gold pieces" in the game as the only coin to barter becuase then developers don't have to worry about exchange rates, extra layout slots, etc. And just because it's called a "gold piece" doesn't mean that it's 100% pure gold. It could be lead with some gold wrapped around it with a stamp.