Its mostly about memory savings, yes. The metalness workflow isn't more accurate or anything like that. Though some feel there is a benefit or simplification in the workflow when you author the diffuse and specular content in the same map. If your texture is all metals or all insulators, you don't need to use a metalness…
In the metalness workflow, diffuse + specular = albedo map (which defines the color regardless of material type). The metalness map itself defines which areas are metallic and which areas are not (generally pure black or white values in the map). The last thing you said is pretty much correct. Pure metals reflect nearly…