If anything it would put a value on "a favor" in general.
I'm not so sure about that. I don't trust anybody else's favor IOU's. If I have a molecular FAV IOU, I can ask you to honor it in exchange for a favor, same as anybody else who holds it. If somebody else trusts you, I can ask them to do something in exchange for my molecular FAV, which they could then ask you to honor. But that's very different from having a balance between the two of us, for which I would have to do a favor in return.
In a related problem, it seems like the issuer of a FAV is very important, and different FAV's are far from interchangeable. Let's say I save the world, and Barack Obama says "Thanks moocowpong1, I owe you one. Let me Ripple you a FAV." Unfortunately, I haven't extended trust to him yet, but I do use FAV's to keep track of yardwork, beers, and other miscellaneous favors with my neighbor Joe Schmuck. Ripple finds the path POTUS -> Joe Schmuck -> me. I end up with Joe Schmuck owing me a beer, and Joe can call in a favor at any time from the leader of the free world. This doesn't seem quite fair.
It seems like there are times when payments shouldn't "ripple". It's great for interchangeable and fungible commodities, but not necessarily things like favors, hours of work, and sex coupons. I'm not entirely sure what the right way to fix that is.