Unity 5, PBS Smoothness (Roughness) texture slot?

Why is there no texture slot in smoothess(roughness) of the new PBS?
I would guess it has to do with performance? It is kind of a big deal designing surfaces to map this property with a texture.

http://www.marmoset.co/toolbag/learn/pbr-conversion

 

The smoothness is in the alpha channel of the metallic texture.

It only needs a greyscale, so instead of a full map only uses a channel.

Kind of like transparency is the alpha of the diffuse...

 

Ahh just saw that now when i hovered over the metal property.
I guess that saves a few bits, but hovering over the small slot maybe isn't the best intuitive solution.
Thanks!

 

It works, thanks!
And we can also use Albedo alpha as an alternative.
But we have all the trouble of editing the channel in photoshop, making the alpha from roughness.

So, Unity people, as an improvement suggestion...
Would be much easier to keep the roughness slot on the standard shader. If present, you can hide Source and use roughness.
Then when exporting the game, Unity could compose the roughness into the metallic or albedo alpha, which one is used. If none, create a white one with that alpha.
No extra texture will be exported to the game.