Kind of a nice addition.
So far I have just been using auto paint as a guideline to paint over the real layers.
After which you have to remove the auto-paint because by design it just adds extra complexity without any benefit.
Note that doing just this is still ideal for performance.
Multiple Single layers only add complexity when you manually overlap them. An auto material is just more complex of a shader no matter what.
BUT, you can probably afford the cost difference if you have a 100x100km terrain to paint on your own…
It comes down to whether you want to publish in this century or not.
Having the ability to specify the physmat without having to work around it using a color output is great. It means MUCH less of a load on the material to do the basic stuff 