Since the first episode of Lost was presented to us the leaders' importance. We saw so many characters to occupy this position. Jack, Locke, Kate, Ben, Eloise, Widmore, Sawyer. A position that was also the cause of major conflicts during the series. Jack vs Locke, Locke vs Ben, Ben vs Widmore, Jack vs. Sawyer. Locke even manipulated Sawyer to kill his father to reach leadership. So... are not the leaders important anymore?
Yes, Jacob and MIB, or next Jacob or next MIB, are clearly important. But, they are the God's figures, the leader is who will have to govern the island. A leader, as shown by Flocke, is not obliged to obey the gods, however he can not allow that "divine" anger harms his people. He must feel responsible for these lifes' fate. The gods (on the island) do not care about the humans' suffering, they just want to achieve their purpose
Having a leader must be very necessary, at least for the Others. Richard spent several years on search of the next leaders of the Others. In 1956, he visited the newly-born John Locke. In 1977, he saved little Benjamin Linus's life and made him one of the Others. In 2004, he helped Locke to become a leader by giving him the Sawyer's. Why so much work?
Sometimes, Others seemed frightened and lost as they long for a leader to finally free them. Widmore and Ben failed, because they put their personal interessents and obsessions first, and perhaps never figure out how really do their work. I do not know, too. But i believe that a leader is crucial for the island and its people future.
This is nothing great, not really a theory, only a big doubt!