It's a really good option, but in my case I'd like to "switch off" placeholders from default skin completely even if they are used in animation timeline in order to override them with multiskin approach.
Let's assume that I have a slot "backpack" in my character. I'd like to see some sort of default contour backpack placeholder in editor and animations, but after exporting I'd like to get an empty attachment in this slot. So if I have to draw a character without backpack I simply should not apply any backpack-variants skin and by default it will remain unset.
But current flow of spine multiskinning will fill unset backpack slot with value from default skin. And I have two possibilities to override it
create skin with transparent placeholder that will overwrite default value or use empty placeholder image in default skin, but this will also hide my backpack from me while animating it.
I think this problem can be partially solved by adding no-export flag to the skin! I.e. I can use composite skins for animation setup and finally don't export and don't use them.