It would be nice if the authors would evaluate how the hardware clocks compare to their software method in the case of maintenance regimes. I understand that their method is "good enough" in the "stable" state across the datacenters spread out through the world. The question remains if the existence of the hardware clocks simplifies the "unstable" states (that occur not when everything runs according to your software design, but when you maintain the infrastructure at that scale) and I believe it still does. I don't have proofs, but my belief is that using GPS and atomic clocks at that scale (GPS clocks on different physical locations) isn't an example of doing unnecessary stuff.