The main idea behind the paper's use of the Kelemen-Szirmay BRDF is physically based specular. Since you already have a pretty good physically based BRDF, I don't think there's any need to use the KSK model for skin (especially if you're using deferred). A PBR model is all you need.
The only reason behind it is that skin has a lower specularity when facing it directly, but at grazing angles it gets much brighter. Basically, fresnel is very important to achieving realistic skin.