Not a bad idea - but not worth the money to me, even when my XF was a daily driver. In 37 years of driving, only TWICE have I had a failure that took the tire down to zero pressure quickly, and in those two cases (the tire was lacerated) I doubt the run flat would have helped much.

I have had only one nail-like puncture in an XF tire in 90,000 or so miles of driving, and I fixed that without even taking the tire off the car with a "tire worm" plug kit and the air compressor.