Must be Seeing things.... by IAmJacksLiver

I remember reading some post about Damon/Carlton acknowledging 3 distinct reasons for the ghosts we had seen in the show. While i do not remember the exact labels for these groups, I do remember the content.

Heres my take on those groupings as of today.

I think that we are seeing 3 things project visions here. MIB/Smoky, Jacob, and the Island itself.

MIB: Yemi, Alex, Christian (raggs), Dave, etc.
Jacob: Kates Horse, Christian (suit), Ben's Mother, etc.
Island: Dreams, Whispers, Walt (wet and tall), etc.

I will try to dissect only Jacob and MIB here.

MIBs visions all fit certain criteria. Body exists on the island, Not buried in-ground. These are tangible items from the immediate landscape. Some of these vision even lead to/or tried to lead our beloved characters to death.
Jacobs do as well, just different criteria. Not on island, Taken from memory (mostly pleasant). These are projections of memories from outside the reach of the island. These visions provide hope, resolution and answers, and good-old-fashioned happiness.

Most importantly lets look at Christian as he can be attributed to both MIB and Jacob. Now, we have seen 2 distinct versions of Christian. I believe that the versions we of Christian walking around in his burial clothes is Jacob. However, in his island-native rags i believe he is MIB. This is consistent with Locke now walking around the island wearing the island gear, and not his burial suit.

Although suit Christian did nearly lead Jack off a cliff and to certain death, i do not think that that was the intent. As we've seen, Jack chasing him lead to the discovery of the caves (water and shelter, two necessities in the wild).

Suit Christian showed up as the FDW to persuade and provide emotional support to Locke as he struggled with a mangled leg to reset the wheel. Christian could have helped him, but Locke needed to do this on his own (sound familiar?) He also gave Locke the ammo he needed to persuade Jack into returning (Say hello to my son for me), a message Locke delivered and Jack felt so strongly about he purchased a plane ticket in hopes to return to the island.

It was rags Christian that lead Clarie to the Cabin and infected her, and lets not forget abandon her child. This Christian also put in the head of Locke that the island needed to be moved/hidden which lead to the time-skipping season and a whole bunch of unpleasantness that followed.

In short, MIB and Jacob have been projecting themselves to the losties since day 1. MIB can only take from what is immediately available to him and aims to cause despair. Jacob can draw on memories and provide resolution, answers, and hope. Jacob it would seem genuinely cares about our losties survival, and MIB wants to see them suffer or die, or both.