Most likely not for atomic clocks
Although you _can_ connect an atomic clock, and rubidium atomic clocks are now _very_ cheap, it's more likely they will just get some sort of central time source, like a cheap GPS disciplined oscillator and use some coax to distribute it. That way you can easily get into the sub 100 ns range when it comes to synchronizing bits of equipment. It probably doesn't matter if the latency is 500 ns or 600 ns, as long as its the _same_ for all people.
So having a sync input makes sense in such situations, even if they will probably never be connected to high accuracy clocks.
Of course you can always set the time by your wristwatch