Right now I'm seriously leaning towards the conclusion that neither Jacob nor MiB are good. Assuming what we learned in Ab Aeterno is true, here's what's happening on the island: Jacob is there to ensure that the "cork" (the island) remains in place, so that the evil (MiB) can't escape. As long as Jacob is around to accomplish that goal, the world is safe.
Now, it's evident by now that Jacob won't die under normal conditions. He'll live indefinitely unless a mortal (not MiB) can be tricked into killing him. So essentially, all Jacob had to do to save the world was to keep the island completely isolated and free of people that MiB can talk into working for him, right?
Well apparently Jacob decides to do the exact opposite. He brings as many people to the island as possible, creating countless chances for MiB to recruit one that will kill him and subsequently escape. For what purpose? What could possibly have been important enough to risk the very safety of the world?
I think Jacob simply hated his job, and was trying to get someone to replace him. It can't simply be a precaution of having a successor in case he died, because MiB alone can't kill him. Jacob just wanted to leave (like MiB), and he didn't care about the fact that he might be dooming the entire world.