Although clock holdover might seem like a pointless feature - GPS outages are very unusual* - what holdover is mostly for is giving you time to respond if someone clumsily knocks your antenna over, or crushes your coax cable, or it gets damage that makes it fill with water, or something like that.

A lot of the demand for high-precision clocks is for cell phone base stations, where there's no guarantee there'll be someone on hand to make repairs promptly.

* They happen occasionally, of course.