If your clock is mission-critical, it should be rather well kept. Quartz, NTP, GPS, the list goes on and multiplies like rabbits (redundant this redundantly connected to redundant thats and backed up by those...). But indeed, sometimes you don't need sub-second precision, and relying on The Grid is useful.
I only have some experience with lab clock sources, but even those have some amount of redundancy by nature. They are generally a very good oscillator (e.g. a very good OCXO) disciplined to a reference with much better long-term deviation (e.g. DCF77 [uses a PM carrier, not the timing signal], GPS, a rubidium source or a miniature caesium source). Short-term (minutes to hours) is governed by the oscillator, and all these sources will enter holdover when the reference does something stupid. More recent ones can tell you the error margin during holdover. Distribution amps can have redundant and cross-checked inputs for independent sources.
