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the face that they can send money across border in 4 seconds and save banks 60% on processing fees, thats why

and by the way it will be traded between all fiat pairs, why are you saying rarely? you obviously have not done any research on ripple

Sure, but banks can also transfer fiat over ripple, cross border etc. If a US bank wants to settle funds with a bank from an obscure country, they might not have USD to x country fiat trading pairs with enough liquidity. But they have usd to xrp, and xrp to x currency. That is what xrp is for. Is there any other reason for them to use xrp? Honest question.