AFAICT, currency is redundant with unit and should therefore be dropped. There are lots of currency-like use cases not covered by ISO 4217 such as gaming applications (e.g., an economy of gold/silver/copper pieces or a magic system with convertible kinds of mana), and also cases that call for compound units consisting of multiple currencies (e.g., EUR-per-USD).

If distinct physical units that measure the same dimension are expressed via unit (e.g., meter vs. kilometer vs. foot), then the same should hold for the more abstract units of which currencies are an important but not unique example.