There are currently great advantages in using projection onto real surfaces for shared ambient user interfaces. Participants are able to see literal reality, rather than reality transformed by any sort of head worn display.
There is a direct connection between us and the real world around us. I think this fully engages our brains in a way that any sort of head worn display cannot yet achieve.
But there will be a crossing point, as technology advances, when something that you can put on like a pair of glasses will allow you the same capability, without the need for a physical projector in the environment. This will be a sort of inflection point, whenever it occurs. I wonder whether there should be a special term for that transition.