I tried something like the OP is doing using curves when I created the microroughness nodegroup, but decided it was too hard to control and pretty un-intuitive.
Through experimentation and input from MartinZ, we managed to come up with a nodegroup that controls microroughness using just a couple of parameters.
I put the nodegroup up on Blendswap. All you need to do to make this work with any shader (including principled) that you want the microroughness effect is to plug the microroughness node group into the roughness slot.
These images show what various materials look like without and with microroughness. The first image just has out of the box roughness value. The second has the microroughness node applied, with the same base roughness value, but with the microroughness effect active (except the floor). One advantage it seems to confer is that you don’t get the bright halo around the edges of higher roughness materials (this is particularly prominent in the ‘black rubber’ material on the right hand pawn).
Through my own observations - I think this effect is present on most real world materials to a greater or less extent (I have certainly observed it on the majority of materials I have taken the time to look at - everything from metals, plastics, paper, wood etc). Contrary to what some posters seem to suggest that his is a very situational/niche effect, IMO this type of effect should be enabled by default wherever roughness is used (with the ability to turn it off where you want the rough halo - like dusty materials).
