I dont understand how ripple prevents spam since XRP is virtually worthless.
XRPs must be scarce to prevent transaction spam
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Users will prefer XRPs as part of a trade over any IOUs
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...there is already a robust trade in XRPs for other currencies like Bitcoin, US dollars, and foreign currencies...
Do you call this "virtually worthless?"

If you control the issuance of a currency, you can control the exchange rate. Bitcoin is popular precisely because no one directly controls the issuance of the currency. Ripple is troubling for the opposite reason.