I have a site that converts in both USD (banners) and EUR (shop).
It would be useful to be able to track conversions in different currencies for a site.
It would be, but currency conversion would be tricky (we'd have to call webservice to know about currencies), which is do-able of course, but not priority at this stage.
It would be better to maintain several balances, one per currency. Currency conversion doesn't happen in realtime, it happens when funds are transferred from one account to another.