here's a question.
Is there a case to be made that a specular map made for an item, like for example this manhole cover , could also be used connected to the roughness channel ?
After all, the rough parts of a metal surface may also be worn smooth and therefore expose the underlying bare metal.
please see screen grab attached as well as an .orbx file to play with.
Try disconnecting the specular map from the roughness channel and replace it with just the greyscale node next to it in order to see the difference in the render.
This does show up more with the north offset slider set to the extreme left or right.
try altering the north offset and the time of day to also see the difference.