It doesn't combine easily with their other techniques for reducing latency and judder. A variable refresh cause problems for low persistence as you'll get variable brightness. G-sync/Freesync is more suited to full persistence monitors where framerates as low as 35fps look pretty smooth when timed with the refresh, allowing you to crank up the graphics and have hugely variable framerate with no discomfort. Such framerate variability (and going so low) would not be comfortable in VR, no matter what you do with the refresh.
It's almost a different philosophy - G-sync/Freesync encourages you to push eye candy higher and higher, sacrificing framerate that would look bad on a conventional monitor. Whereas in VR, it will always be about maintaining a high framerate, sacrificing detail if necessary. If you're able to keep the framerate high, then asynchronous timewarp should be enough to deal with the odd drop here and there, while keeping the refresh (and the low persistence) perfectly consistent. Perhaps it can work together, but it's not a big win like it is for monitors.