Here is a theory I have.
First of all, Jacob vs MIB, a game with predefined rules. Both seem to have knowledge of events that will unfold, both have been around long enough and seen such similar events unfold that they can very much predict the outcomes. This theory ties in with Sayid, at the beginning of the season we watch our protagonists bring Sayid to the temple, when Dogan reads the note left by Jacob, it informs them if Sayid were to die, very bad things would happen. We then see Sayid die, then come back as the character formally called Sayid (lol). As viewers most of us think, ohhh noo, Jacobs plan fell to shit. We watch as he aligns with Lock and seemingly becomes evil. Let us have a look at Sayids plot line for this season; he kills Dogan and his translator, he gets desmond, he is told eventually to kill desmond (we will break right here, our brother Desmond sais a few words to Sayid which seem to resonante his sould back from one side of the spectrum to the other), he then tells Flocke ! that desmond is dead. Now, he kills the power on Hydra Island so MIB can do some badassary, he gets on the sub. On the sub, he redeems himself by grabbing the bomb and running, assuring that not ALL of our candidates get killed in action.
So, let’s just think about this for a second, if Sayid didn’t die, turn evil, join Locke, find Desmond, not kill Desmond, bring jack to the hydra, get in the sub, and sacrifice himself. ALL our Candidates would have died in that explosion.
Lost is a game played between two sides, one good, one evil, but let’s not be fooled for a second by this MIB looking all omnipotent, he may see ahead 2 moves, but Jacob can clearly see ahead 3.