Well, ok, it's not that black and white. But the targets all need to be the same size and there are already 3 targets used. (Not taking DirectX 10+ only into account.) So it will get a little tricky to store three 16 bit channels given that the first three targets are 32 bits. It's not impossible, but it's not as simple as "FP16 when HDR". Yes, you could call it a "metallic workflow". I've seen some BRDF document with a metalness parameter before I think. My take on it is that we mainly do architectural visualisation and (almost) everything we need to show either reflects in the diffuse color or white. So it simply saves room. With games in mind this might be a bit limited though. I can imagine many game designers are just used to the fact that there is a separate specular color. And I also like the idea of being able to store more realistic metal reflections or for example thin film reflection effects. In my case emission was rendered additively in forward mode after the deferred pass. Eliminating any precision limitations with the emission intensity. The down side is similar to prepass lighting in that you have to convert the geometry twice. But the advantage is also similar in requiring less storage space in the g-buffer. Of course this design didn't take any realtime GI into account.