If EYE cant have it no one will... by Grimlocke

It would appear there are rules in place designed to keep a game in order. I believe neither the MIB or Jacob can kill any of the candidates as well kill each other. It would also seem that they cant explicitly tell the losties what to do or not to do.

Jacob seem determined to convey to Hurley the importance of Jack going to the lighthouse. Also, it seems that either Jacob has future knowledge of what losties are likely to do (perhaps because of past iterations) are he knows these people well enough from watching them what they probably would do.

I believe that Jacob knew Jacks reaction would be to smash the lighthouse mirrors. I don't have direct knowledge of what the device actually was intended for but it seems plausible that such a machine could be detrimental if the MIB could utilize it. Perhaps Jacob simply didn't want this to happen, and NUDGED Jack in his own way to do what he thought Jack was capable of doing without directly telling him and keeping the rules of free will in play.