There's two possible scenarios. (1), you've got a setter who knows what he's doing, and he notches the prong properly so that it doesn't rest on the knife edge, and is stabilized against the diamond above and below.
In this scenario, the prong protects the knife edge portion of the girdle.
In the second scenario, the setter notches the prong improperly, and has the prong resting against the knife edge.
In this scenario, the first hard knock that prong takes will possibly result in a shallow chip running down from the girdle into the pavilion area. Sometimes they run all the way down to the culet.
There's a lot of setters about who don't have enough sense or expertise to do this correctly, so you want to make sure you've got somebody that knows what he's doing setting the stone. Discuss it with him or the manager beforehand, to see if they understand the concept.