Recenlty played around with skin tone setting using an older female model.
Hoped the resut would be a softening of some wrinkles.
I could see not difference at all whether the setting was off or on.
It's extremely subtle. The effect depends to some degree on what your detail setting is too -- if you're running at Detail Lvl -7, then Skin Dtl won't do anything. If you're running at Detail Lvl +7, then Skin Dtl will have the most effect. But even at its most, it's still very very mild. Too mild to be of much use, I think.