Interesting that they chose to implement Favors as a unit, rather than as a resource, but considering how they explained their development I can sort of see why. A unit can keep track of who it was originally from -- all those Workers that you can capture from other civs, or liberate and give back to other civs, for example. So I can see how making Favors into units allows you to track where the Favor came from, as opposed to just making it a resource that wouldn't carry any such information without additional coding...
Unit as a "unit of measurment".
And it seems like a cool concept. Especially if only the attacer looses the Favours, not the defender.