I can't believe it took me so long to figure this out. Most people think that MIB is Locke because Locke is dead on the island. I think he may be Locke because Locke is a common component of everybody's memory. Therefore, he can manifest himself as Locke to all of the people on the island. That is also why they needed to do all this time travel with Locke so that he would meed all of the common players (e.g., Widmore, Hawking, etc). That is also why the writers also made Christian a memory of Claire and Sawyer.