You mean the built-in Diffuse_Color/Value and the built-in Specular_Color/Value, which are basically the Diffuse node and the Specular node? I never use the Specular node (built-in or separate) so no to that, IMO. I always use Blinn.

For the diffuse reflection, it is a matter of roughness. If a surface does not exhibit powdery roughness like clay or the moon, then the benefit to use the Clay node isn't there, simply because when you set Clay roughness to 0, it is identical to Diffuse. In such cases, I use the Diffuse node or (rarely) the built-in Diffuse.

I use the Diffuse node, not because it's different than the built-in diffuse, but rather because I need to interpose something in front of the Diffuse node almost all the time, such as a Fresnel_Blend node.

It is possible to Fresnel_Blend the built-in diffuse, but the extra wiring involved isn't worth the effort.

If a surface exhibits subsurface scattering, then of course I use the Scatter node and not the Diffuse node (built-in or separate).

FYI if you make the scale of a Scatter node really high, it produces the same effects as a simple Diffuse node does. Scattering exists in all real life materials, but in some cases the scale is so short that it appears to happen at the surface, and so is sufficiently modeled by the traditional Lambertian reflectance equation.

In that case, using a scatter is extra computations for no benefit.

 

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)