Has anyone thought of the Twilight Zone episode "The Howling Man"? I know that if you look, you can find a correlation between Lost and almost any movie, TV show, or book, but I think this one may be worth mentioning -- if only to amuse myself until the next episode.
A man has Satan trapped in a room barred only by a staff (the "staff of truth"). Since his imprisonment, the world has been peaceful. A newcomer arrives and is tricked by Satan into letting him go (removing the staff). The man realizes his mistake and the man who had been keeping Satan tells the him that "man's inability to recognize the devil has always been Man's greatest weakness". He spends the next few decades trying to find Satan so he can lock him up again, but when he FINALLY does he finds he cannot watch him 24/7... the maid slowly removes the staff and opens the door... starting the whole thing over again (for a 3rd time - that we know of).
What if MIB is like the devil and Jacob is like his keeper? Sawyer sees no reason not to help MIB leave. Only a circle of ash (staff) kept him at bay. Jacob can't watch him alone 24/7.
Probably just another wild goose chase, but I find the perspective interesting.