I have always been of "The Wizard of Oz" school of thought when it comes to Lost. And in Ab Aeterno, my favorite line was when MiB told Richard that the only way to get of Hell, was to kill the Devil. It reminded me of when Ben, um, I mean the Wizard (they look so much a like) tells Dorothy that in order to go home, she must kill the Wicked Witch of the West and get her broom stick. Of course, we find out that it was a lie because Ben, um, sorry the Wizard, lies all the time - and he was just trying to make the Munchkins think he knew the Leader. And really, as Juliet, um, I mean Glinda (they look so much alike, too), tells Dorothy, she could have gone home whenever she wanted - she just needed to know WHY she wanted to go home. And then Dorothy goes home to her Flashsideways world, where everything worked out for the best...or did it? According to Gregory McGuire's book Wicked, it really didn't all work out well in the end. Oh, and by the way Gregory McGuire also wrote another novel called, um, "Lost."
Anyway, I'm really going off on a tangent, here. To get back to my theory, or thoughts really, was OZ really the "cork" that kept Dorothy from experiencing the evils of the world? Or was it the place that gave her the moral tools to go back into the real world and make good choices on her own. And if Dorothy was kept there forever, how would her time in Oz ever really pay off? Jacob needs to let go, and have faith in mankind. MiB is not evil, rather he is temptation. It up to mankind to learn from its mistakes and resist temptation - when it's appropriate. But they can't do it in a vaccum, or in a bottle or in a fantasy island world. In order for it to mean anything, mankind has got to prove themselves in reality.