I think you have to look at superfat in soap as filling two functions -- one of adding mildness and/or reducing the irritation factor of soap and the other safety issue of ensuring soap is not lye heavy. I think unsaponifiables can fill the first purpose, but they cannot fill the second because they do not react with lye.
An example of adding mildness or reducing irritation would be the unsaponifiable waxes in jojoba, lanolin, and beeswax. They form a protective barrier on the skin to reduce irritation and moisture loss.
I can make a good argument that these ingredients might provide more benefit in a leave-on product (lip balm, lotion, etc.) than in soap, but the fact remains that the unsaponifiables may give enough benefit in soap to make it worthwhile to include them in recipes -- as you, Effie, have learned from your most interesting shave soap tests!
I wouldn't assume all unsaponifiable chemicals increase mildness or add other skin benefits. A person would have to dig into the chemistry of the fat to know more about that.
