The value inputs multiply the corresponding colour input and pass that value on to the next section of the PoserSurface internals when rendering.
There is some "magic" associated with the Diffuse input for the preview in that it will pass through texture colour even when the Diffuse_value is set to 0.0

Quote - Now, to my knowledge there's no such thing as plus or minus 0... zero is zero

The concept of signed zeros does exist in the "One's complement" system (-> http://en.wikipedia.org/wiki/Signed_number_representations#Ones.27_complement).