In a "realtime system", sure. But for almost all workloads programs have to deal with overshooting. Like, maybe CPU overloaded or swap intensity caused that second to "disappear". On almost all workloads you can't just assume that you'll execute on every single second.
And if you do need to be scheduled on every second, well you always needed to use a monotonic time.
Negative leap second should only cause measurements to be off by a second, at worst. Added leap seconds means time goes backwards, which is worse.
