I just had a thought that I haven't heard discussed before.
We've heard references to a type of security system, which seems to be the smoke monster.
We know the MIB (smoke monster) wants to leave the island.
We know Jacob doesn't want MIB to leave the island.
Widmore has said that if MIB leaves the island, everything they knew and loved would "cease to be."
It has been easy to assume that if Widmore is correct about MIB leaving the island, that the "ceasing to be" would be caused by MIB. What if MIB is not the cause for everything ceasing to be, but is only the security system to keep a much greater evil (that we have not yet seen, and that would itself cause the "ceasing to be") to be trapped. MIB is either apathetic about the results of this evil escaping or is just tired of being the security system, and wants to leave the island. Jacob wants to keep MIB on the island so that the real evil does not escape and cause everything to cease to be, but is instead continually kept in place by MIB.