The harbinger of the companions also oversaw the joining of new members and handled disputes, yet even when 66% or 75% of the circle (depending on your choice) stop being werewolves that's not something that causes a dispute or even an official change in policy.
The way I understood it, Vilkas and Farkas left the Circle once they cured themselves of lycanthropy. The Circle was just a group of Companions that opted to become werewolves. The group itself wasn't anything special beyond that, other than that it happened to include a number of senior members when you joined (not leaders, the Companions haven't had a leader since Ysgramor).
That's partly why I didn't mind becoming Harbinger, because it's not a leadership position... you're a regular Companion that has a certain amount of respect and dishes out advice (though it would've been nice if members would actually ask you for advice sometimes).