As far as UI goes I can see it being a keyboard to your mouse. Some UIs like menus are actually easier to design and interact with in a 3DOF plane.

But as far as hardware goes 3DOF is just a stepping stone; we only use it because it's easier to make. Once good, efficient 6DOF is available we can pull 3DOF out of it whenever we need to.

Assuming rough parity in terms of simplicity and cost, no. Unlike, the limited range of B&W photography vs. color, there is no intrinsic value in having a compromised sense of space and severely limited range of motion when using immersive media.

Other than cost to produce, not really and you can always ignore position or rotational tracking to drop down from six to three which will likely be the default when losing tracking from an inside out system.