I wasn't saying because of PBR. I was saying *before* PBR. I meant to say that you don't have to do it now with PBR workflows. Energy conservation is now a property of PBR shaders so you don't have to account for that in your maps now. Sorry if I implied otherwise. EDIT:Or I am mis-understanding you and it's not due to the…
Well you are right in saying that with PBR it's no longer a concern; roughness maps are indeed monochrome only. However, the Linear-space lighting thing is something that was figured out before PBR became all the rage; the article Eric linked is from 2010. Tells you all you need to know about it.
Totally. You may only want to constrain yourself to the laws of PBR if you are attempting to create photo-real art. However, deviation from reality is part of being an artist!
The basic gist of it is before PBR you had to account for energy conservation in you maps. Energy conservation basically means no more light is leaving an object than it received. So because is so orange you would put blue in the specular to balance the amount of light exiting. The link above explains it well.