I think the usability of spatial controllers is pretty tightly linked to use of a VR viewer, since it's the HMD that constructs the "space" for the controllers

I think the usability of spatial controllers is pretty tightly linked to use of a VR viewer, since it's the HMD that constructs the "space" for the controllers to work in. I don't think Tilt Brush would translate well to a regular monitor with spatial controllers. I can see some potential with 3D displays, but I think if spatial control enough was a paradigm shift then the motion controllers we've seen in gaming applications previously would have translated to professional applications the way VR is now. VR gamers using traditional dual-analog style controllers aren't leveraging the full spatial immersion of VR, so they don't report a spatially immersive experience. When I describe "VR as HCI" I mean everything involved in immersing someone in an altered reality, and the HMD is only part of that. I think it's more apt to say "XR is a new form of HCI," to include tech like the Hololens, but that terminology hasn't really entered the common lexicon yet. VR and XR as a whole give unprecedented access to the primary I/O pathway of the human: our eyes and our hands.