It's kind of interesting that no materials have metalness between 0 and 1. It might as well be a boolean or a tag.

I know it doesn't physically make sense to have it in between, but then I wonder why that ability is even included in a system that supposedly should help you make physically correct materials.

Maybe metalness of 0.5 could be used to describe a fine mixture of 50% metal and 50% non metal?

Maybe it's needed to provide an alias of sort when transitioning between a metal and a non metal in a texture