Due tothe physical law of Conservation of Energy, (diffuse+specular+sss ) = 100% total. Bowing to richard, SSS ought to take the absorption into effect.
This ain’t the real world.
Would it be nice if the shader, somewhere, took this into account and did the balance for us? Yes.
Are the results accurate? Well, no not really.
SSS is actually a much larger part of the visual appearance of many things, human skin being a particular example of where it is very important. Diffuse reflectance is actually pretty damn low on the percentage of what we actually see. But what we have here in Studio and other renderers is not a physically accurate representation: its a best guess, an approximation, a replica.
This is also ART. Some of it really OUGHT to be up to the eye of the artist, attempts at photo realism somewhat to the side.
Until Studio has a shader like ezSkin (yes, I know, its not a shader per se; but you know what it does have: conservation of energy built in, or at least an approximation there of), Studio is going to be more on the artistic interpretation side and less on the realism side.
And I’m not dissing Studio, you’ll find me in the Studio forums over the Poser forums 9 times out of 10. These are simple facts.