Since specular has no effect on metals, using it is… useless.
but if the material isnt a metal, its not useless.
Also, its a basic guideline, just like metal is, its up to you (the user) to play around with it and get the result you want.
There are no rules, just guidelines.
If your specular map gives you a good result, then why not use it? 
I personally often forego a specularmap and use a grayscale version of the albedo and multiply it with a value of somewhere between 1.1 and 4 depending on my needs. (Again, its all in the eye of the beholder (or user) to get the desired result we want.
Think of it like a coloring book, we learn to draw inside the lines, but drawing outside can give great results as well if we dare to let the guidelines go