I think there's an intent to keep the physical material definition pure - only a rough surface will lead to rough (more diffuse) transmission. You can't have a non-glossy 'internal matter' without a rough non-glossy surface per definition, and vice versa, you can't have a clear perfectly transmissive 'interior' with a rough surface. In other words, the surface finish defines what happens internally, it is the terminal that sends ray through the object. The ray enters and travels through the object depending on its surface properties.
That's why this definition needs a coating layer, which they provide. But it has to be an individual layer of a coating material and must not interfere with the aforementioned principle.
If I understand correctly, that is...
The current shading model doesn't account for other effects, such as varying dispersion, varying IOR within a medium etc, too, but we don't care because it's really a tiny fraction of the materials that Corona is used for in a typical situation.
Please continue to provide examples for something that you can't do with the physical material. I'm sure that it's going to help if it's a valid request.