I think Jacob appears to various people on the Island as a vision of their dead loved ones.

Richard first takes an interest in young Ben when Ben tells him he saw his dead mother. Richard thinks that makes Ben "special". The dear departed also appear readilly to Jack (who's mistrustrfull and ignores visions of his father) and to Echo and to Locke.

In the episode where Echo dies, Yemi appears to Echo. Echo first thinks that he's seeing his dead brother. But when Echo fails to express remorse over his past misdeeds, he suddenly realises the vision is not Yemi but something else. He becomes enraged, then frightened, then the monster comes in and kills him.

Finaly Locke also sees the dead Boone. But Lock's visions of the dead come with great difficuly after spending time in the sweat hut. He does not see Jacob at all the first time, an outcome Ben has apparently expected and wanted, because when Locke asks him why Ben shot him, Ben replyes "Because you heard him".

Further, Richard does test the Lock as a young child, but is clearly disapointed. To me, this indicates that Locke is a bit of false prophet. Richard's disappointment in Locke foreshadows Lockes failuire as the Leader of others.

I think there are enough hints that it is Jack who is meant to be the leader of the Island, not Locke. In the episode where Jack goes to Thailand, he's told he is meant to be a leader but he does not want this role and it makes him angry. Jack's visions of his dead father are really visions of Jacob.