A few different posts have kicked some ideas around in my head lately. Firstly, people have been talking about the possibilities of there being two Lockes running around at some point, one being the MiB/Smokey manifestation that we've seen and perhaps another in which Jacob does the same thing. I'm a bit skeptical about that, but I've entertained the idea to see what I could make out of it. Here's what I've got.
It seems possible to me that Jacob and MiB were in fact battling over John Lockes body, but not as described above. As some background, I'll put forward the idea that Charles Widmore and Eloise Hawking, as of the day of the Ajire 316 flight, are not necessarily working towards the same goals. When Widmore met with John Locke (and gave him the Jeremy Bentham identity - Bentham being a man in history who was a proponent of individual freedom), he seemed to be quite interested in making sure that Locke got to the Island alive. He gave him all the resources that he would need, and even a bodyguard.
To be completely honest, Eloise Hawking (whose namesake and character suggest a deterministic view of the universe) seemed like she was in on something with Ben. That's how it seemed to me. She was harping on about recreating the exact conditions of the return flight, but didn't seem too bothered that there were people missing. Of course, that's because it didn't matter; rather the only important part was John Lockes body. As soon as Ben found out that Locke was off to see Eloise, he killed him. Ben had already sniped Lockes bodyguard earlier. For some wild specualtion - If Locke has have shown up at the Lamppost alive, Eloise's conviction in her determinist view might have been shaken. As it happens, Locke showed up dead, between her and Ben they convinced Jack to get his body on the plane, and MiB got his hands on Lockes appearance. Was Eloise working for MiB? More likely, was she being manipulated by him? Food for thought.
What would have happened if John Locke has made it to the island alive? He would be running around, acting of his own free will. Here's the little bell that was ringing when I was reading the thoughts about Jacob making his own version of Locke: Jacob is supposedly an advocate of free will. Wouldn't having the living, breathing, John Locke, making his own decisions on the island, be the utmost incarnation of Jacobs ideal?
So then; why the scheming over who gets to drive Lockes body? Suppose the island creates a complex system of causality loops (if you're unfamilar with what I mean, check some recent blogs by Mittelman, DaemonRising and myself to see where I'm coming from) where certain people, through the nature of the time-travel shenanigans, always have and always will be responsible for certain major actions on the island that result in the loops completing. Michael couldn't die before he blew up the Kahana, because he DIDN'T die before he blew up the Kahana. HOWEVER, Michael had all sorts of other choices he could make. He could smoke a cigarette, tell the truth to his son, put $500 on black in roulette, whatever, without compromising that loop.
So, what if these rules can be bent, slightly. If something huge is about to happen on the island, something very huge that also involves John Locke, then it could be safe to say that John does something very important. As long as John Locke is there to do it, then everything is dandy, and anytime up until then he can do nearly anything else he wants. What if (you've probably seen this coming by now) it wasn't just that THE John Locke had to be there, but A John Locke? If Jacob had his way, the real John Locke would be on the island, and would be making all sorts of choices from his own free will up until the point that are probably beneficial to Jacob and 'good'. But if Flocke was the one to do it, then from anyone elses perspective it's no different (as far as any observers are concerned, it will still appear than John Locke did action X) except he'd maybe doing some real evil shit along the way, that would be beneficial to MiB and 'bad'.