Curious About Mid-Gray Metal Values

To my understanding, Unreal expects a 0 or 1 value for the metal input about 99% of the time. Looking at Outpost 23 in UT, I find quite a few examples of materials that use mid-range values for metal, and was curious why. Is this an example of a not-technically-PBR correct material that was done for artistic purposes, or are there situations where a material like this is still PBR-correct?