Because then you have:
1) all the benefits of bitcoin
2) a medium of exchange between currencies.
Ripples can be used as exchange fees between those that utilize "trust" with other friends, but it can also be used as a medium of transfer between currencies.
If the Ripple servers are centralized, it won't matter. The idea of ripple, however, is interesting, and I wish they'd release the source code sooner than later so we can actually do something with it. Or maybe someone could invent a debt-based system with a blockchain detailing what debts are owed to who.