XRPs have all it takes to become a good currency
So is the dollar, the euro, the pound, the yen. Well, until the moment when the corresponding central authorities decided to print new trillions of these "good" currencies. How is XRP different? Oh, the central authority promised not to print more than 100 billion? Is that what makes XRP a good currency?
Um, no.
Dollars are not scarce, the Federal reserve can print as much as it wants. (OpenCoin didn't "promise" not to print more, it is impossible by protocol)
Dollars are not durable, they can be easily lost and stolen.
Dollars are not portable, they need to be physically carried around, and in large quantities have significant physical volume and mass. (I'm assuming physical notes, if you consider bank balances as dollars they are slightly more portable but less scarce and durable.)
Dollars are not as divisible as Bitcoin / Ripple credits.