On the other hand, any publicity about well known (if only by most people through association) leaving not only reinforces the narrative but likely bolsters the reality as it effects people's opinion, so it's important in it's own right because of that.

Not as important as some Kardashian publicly quitting the platform, but important for the same reasons. Public individuals have some small (or large) level of influence over public opinion.

That being said every celebrity that quits helps reduce Facebook's dominance, as their fans may consider quitting too. A good thing IMO.