Standard shader: Glossy (metal) surfaces appear black, if scene has no default reflection set.
Since we're talking on PBR, I assume object behaviour (without default reflection) should be equivalent to one, located in a totally black room, but still with the light on it. If we put a metal object into the real condition, we'll still be able to normally see it - with its colour and light reflecting from it. Current behaviour physically acceptable only for totally black surfaces.
See attached img of the same material with and without default reflection (Reflection set to Custom with no map selected)