I think 3-dof would lead to motion sickness issues. You might be mostly moving your head rotationally, but if you move at all in the xyz directions your movement is no longer going to match up to the image in the headset. That mismatch between movement and vision is the primary cause of motion sickness. Phone VR was an evolutionary dead end for VR tech. The future is 6-dof, hand, eye, face and body tracked, both for work and for play.