I argued at the time of introducing NumberFormatter that currencies should use the .unit() method. The reasons I made that argument still stand. However, feedback from users suggests that the advantages of a standalone .currency() method may outweigh the cons.
Here are some potential advantages to adding a .currency():
1. More discoverable and consistent with what users expect
2. Allows us to add overrides for String ISO code and java.util.Currency
For now, I am creating this ticket as a place to collect user feedback.