Eko and Locke

A couple of days ago someone asked why we don't see Eko anymore. I imagine it is because he has served his role in the series. From what I can gather, Eko functioned as a foil to Locke. Eko, a priest, took his and Locke's visions to be signs that pressing the button every 108 minutes was important. Generally speaking, given the dubious nature of dreams/visions, Eko's decision to act on these messages was a demonstration of his faith.
Locke, on the other hand, fights with his faith at various moments throughout the first three seasons. Although he received the same messages as Eko (or at least discusses them with him) he questions the need to push the button and eventually ignores them. Interestingly, he puts his faith in the second Dharma orientation video he watches in the underground hatch. Although he has no way of corroborating the content of the film, he takes its message to be true. He uses the "evidence in the film as a reason to stop pushing the button and preclude others from doing the same.
It's not until the very end of the season 3 that Locke's faith is finally cemented. The moment occurs just after Ben takes him to meet Jacob. Locke doesn't see Jacob but hears him say, "Help me." Ben, jealous of Locke's connection with Jacob, shoots Locke and leaves him for dead in the pit with all of the Dharma corpses. He doesn't die but appears to faint for some undetermined period of time.
When Locke comes to he's unable to move his legs. This is a classic sign that Locke has lost all hope. Having resigned, he picks up a loaded gun that he finds among the corpses and prepares to shoot himself. All of a sudden, Walt (an apparition generated by the island) appears and orders him to put down the gun. He tells Locke he can move his legs and he has work to do. We don't see Locke get out but we know it happens when he shows up at the communication tower and throws a knife in Naomi's back to prevent her from contacting her people. Because LOST is rife with Biblical references, I can't help but see Locke's departure from the grave as a resurrection. Correspondingly, Locke becomes the new savior of the island. He doesn't take on this role immediately, but his faith is resolute (he took advice from an apparition!) and he is willing to do whatever it takes to save the island. Once Ben leaves, Locke finally assumes his role as Jesus.
So yeah, the reason we don't see Eko again is because Locke doesn't require his spiritual guidance anymore. As you may or may not remember, Hurley plays a game of chess with the invisible ghost of Eko in season 5. Beyond the fact that Hurley is seeing ghosts, I don't think this moment or Eko's part in it are particularly significant.