Am I right to assume that in general, the distance the light moves from the entry point before re-exiting in subsurface scattering is small enough to be unnoticeable?
@jfondrie No, this is not true in general. For example, if I put my finger over a strong light source (the LED on my smart phone, for example), I see a lot of light traveling through my skin; see this image for an example. So there are many materials where the distance between the entry and exit points is small, and these are materials that can be well characterized with a BRDF where it is assumed light enters and exits the material at the same point. For other materials (wax, soap, marble, etc.), the entry and exit points can be quite large and a BRDF does not fully capture how light interacts with these surfaces.