Yeah Unity's texture packing method is really dumb. If you have a 32 bit image, you're using the same VMEM as 2 24 bit images (assuming you have compression enabled), which means you could use a full color specular map, and a RGB map packed with gloss, AO, and a 3rd map (height? alpha? emissive?) for the same cost.
Echo: I'm not saying it has less, I'm saying it's the same, specifically with Unity 5. But specular maps are more flexible than metalness (if you want non 4% values for insulators in the metalness workflow you need to use yet another secondary specular map). Unity's workflow forces you to use a 32 bit image rather than a…