Transactional atomicity is exactly the reason why ripple needs a native currency - the native currency is what provides byzantine fault tolerance[1], just as in bitcoin.

Supposedly, ripple reaches consensus under diverse graphs of validator node networks. But its a research topic still undergoing study (Ripple Labs say they have whitehats closely examining the protocol).

1. http://en.wikipedia.org/wiki/Byzantine_failure#Practical_Byz...