At times it is clear that MIB is who he is...Jacob's nemesis. He discusses the loophole, he relishes the death of Jacob and turns into the Smoke Monster. All the kinds of things you expect him to do.
But at other times, he appears to act a lot like Locke. He recalls the camp at the beach where he first met Ben, he tells Sun he will help find Jin and he tells the boy in the jungle "Don't tell me what I can't do".
So to explain this logically (if that's even possible in Lost) it would seem like MIB possess more than just Locke's physical appearance. He possesses Locke's memories, mannerisms and even personality. Which leads me to ask, is MIB the only one in control here or does Locke have an influence (buried somewhere way beneath) as well?
I think the writers understand the popularity of Locke and would like to give hope that Locke still exists somewhere in MIB. But at this point it seems like more than just a writer's ploy.
On a side note, how many kidneys do you think MIB has?