Affects: 2.79b, 2.8
The specular input of the Principled Shader causes quite a bit of confusion, and I propose renaming it and/or adding info in the documentation.
During a lengthy exchange between me and the author of a recent commercial Blender addon called Super Texture, it came to light that the addon author had misunderstood the nature of the specular input of the Principled Shader. As a consequence, he had plugged non-PBR specular values into the specular socket in his addon. This mishap happened to him due to the fact that he erroneously transfered his understanding of realtime PBR to Cycles. I should add that I myself did not understand the specular input before the exchange, and only came to understand it after some investigation.
Then, a few days later, I stumbled upon another Blender addon, called Blender Addon PBR. In this one, its author incorrectly assumes that you can simply plug the specular map from a spec/gloss workflow into the specular input of the Principled Shader.
These two occurrences lead me to conclude that the specular input of the Principled Shader is in general quite prone to being misunderstood, either as input for specular maps from older, non-PBR workflows, or as input for specular maps from the spec/gloss PBR workflow.
I therefore propose renaming the socket to "Specular Level" (which is the term used in the Substance tools, see here), and/or to add clarifying information to the Principled Shader page in the Blender documentation, so as to prevent the sort of confusion described above. And yes, I'm aware that the name "Specular" comes from the Disney shader; it's just a rather unfortunate naming choice, IMHO.
q:-) <= Quang