There really needs to be a great deal of clarity before LOST ends.
There is much debate on the partial "rules" given by Jacob or MIB to our castaways.
For example, Flocke says he cannot get over water as the smoke monster. He needs a body and boat to leave the island.
But then, some speculate that MIB was Christian both on the island (that's what Flocke said to Jack) and off-island (not confirmed.)
With the symmetry of show story scenes popping up constantly in Season 6, maybe some of the disconnected minor details can answer some of the speculation, or add some circumstantial evidence to the discussion.
Flocke told us that Jacob had stolen his body, "his humanity," and that wanted to kill Jacob for a long time (1867 beach scene). When Jacob "stole" MIB's body is unknown, but it is probably before 1867 death threat. So if MIB was the "original Jacob," then who is really the current Jacob?! And the current Jacob took MIB's body, did he not also gain access to all his memories, thoughts, logic just as MIB has a Flocke with Locke's mind and memories? Does that also mean that MIB's form was really a predecessor of taking Locke's form?
If MIB-Smokey can shape shift into various forms (Yemi, Christian, Locke), why could it shape into the form of Jacob, a body he was once quite familiar with - - - so that leaves open the question of which entity was the "off-island" Jacob.
MIB cursed the amount of time and effort he had to go through in order to get Ben to "kill" Jacob in the statue. Now, did MIB really mean just the psychological stress he put on Ben from early childhood, to the purge, to leadership of the Others, to his abandonment by Jacob after blindly following him? If MIB could not leave the island, then Ben would be his only long term pawn.
MIB also indicted that he needed a "body" in order to form his human appearance. He said he used Christian's on the island. He also used Locke's (but not the actual body). So, the question remains "how" does MIB use a dead body and why a body is needed in the first place. But it does put a spin on Locke's FDW moment. It was ghost Christian who told Locke he had to turn the FDW on his own. He also said not to trust Ben, but go see Eloise Hawking about returning to the island. Christian also told Locke that he had to die (sacrifice himself). When Locke turned the wheel, he foomed from the island, but apparently ghost Christian did not. If MIB was using Christian's body to make the arrangements, why would he need a dead Locke to return to the island? Or take Locke's form when Ben knows he is not really Locke?
Maybe it has something to do with an unspoken "rule."
The only connection between Jacob's killing and MIB's taking over Locke's body is Ben. Ben is the one who killed Locke in the hotel room. It was Hawking that said they "all" had to go back to the island, including dead Locke. (And ghost Christian told Locke about Hawking, a person he never met.) It infers that MIB-Christian was allied with Hawking-Widmore in getting Locke back to the island. Once back on the island, Flocke convinced Ben to kill Jacob. Now, Jacob apparently bled out in human form from the knife wound, but when Sayid attempted the same on Flocke, nothing happened. If Jacob had "taken" MIB's original human body, is this why there is a difference?
It still does not answer the reason why MIB had to take Locke's form instead of remaining as ghost Christian. Especially, if the end game deals with manipulating Jack. It is a footnote that Jack never resolved his differences with either his father or Locke during their lifetimes.
But if the Lost saga ends with the end of the Jacob-MIB conflict, is there any meaning to the fact that Ben killed both Jacob and Locke? In order for MIB to leave, the same person had to "kill" both Jacob and Locke? The smoke monster once tried to drag Locke underground until Jack saved him. And there is some rule that candidates, like Locke, were not supposed to be killed by MIB.