When is it acceptable to use roughness without a map input in the Principled BDSF node? For a PBR material setup, one usually have map inputs for the color, roughness, normal nodes. If a material has an undefined roughness, but it's not perfectly smooth either, is it okay to use the roughness slider as a substitute for a map input? Would such a technique have an impact on the photo realism of the object?
It is perfectly normal to use a single value for roughness in all the cases when roughness is uniform across the surface for example with smooth clean objects made of the same material.