"Anyway, the question remains: exactly who needs to have the leap-second occur as an inserted second, rather than a spaced out smear?"
Any system that has heterogenous hardware and software that nevertheless passes around time stamps, could get upset if different machines used heterogenous sources of time as well, so that this smearing wont happen in unison. That is, different ntp servers some of which smear, others not or they use different smear period and interwall.
Rare - surely, as rare as leap second problems, but have a million systems, and a few gets hitten by this hard.