just read this over at aicn.... kinda love the thinking behind this theory:
Assuming that 1) It's all about the choices individuals make, and MiB and Jacob can only influence them and 2) that MiB is some kind of Demon. He hates humans, wants to kill them. Jacob has the job of keeping locked away, on a deserted island, so he won't hurt anyone. Jacob is tired of this situation but cannot leave unless being replaced by a good candidate. So he goes outside the island, influences people into making choices bringing them to the island. Once on the Island he watches them go through some "tests" to see if they are worthy or capable enough for that job (or if they make the right choices?), crossing the names that are gone until there is one left. Now when MiB says that "there is nothing to protect" about the island, he lies. The island is there to hold him prisoner and should NOT be found, because that would give him so many opportunities to escape. So Ben wasn't lying when he said that if Widmore found the island, they were all going to die. It would litterally be the end of the world if MiB left the island, hence why the gang protecting Jacob call themselves "the good guys".
Assuming that MiB would only be allowed to leave if Jacob set him free : Since the rule states that you cannot kill the candidates - that's what the boy said - on the island, then one of them had to die outside AND COME BACK, therefore provinding the loophole for MiB. The loophole is : MiB arranges to have Jacob killed (again influencing the choice made), has someone replacing Jacob (like Sawyer) and that new Jacob willingly sets MiB free.
But then is Ben on MiB side? He did kill Locke, he did insist that Locke had to be on that plane, he did unleash Smoke Monster when his daughter got killed, he did round up the people "on Jacob's list"...