Why complicate something just for the sake of uniqueness? There's a bazillion alliances and if each started floating its own currency, you'd soon need a degree in foreign trade to understand what exactly is profitable from which alliance. Also, imagine working hard for months to save up for something and then your alliance leader does something stupid, your economy crashes and your hard earned savings are worth 3 bauxite.
Let's not complicate a game with real world mechanics understood only by experts in the field.