We know that John Locke's body was buried on the island. MIB didn't actually possess it - he just copied it. So when the light / power went out in the cave He and Jack both lost their imortality.
So it seems to me it would have been cooler if at the moment he realized he wasn't Smokey anymore his fake Locke body should have just desolved into black smoke and dissipated.
After all, his real body was back in the caves with mother. (Or did the survivors bury the skeletons?)

(Yeah, I know! For the sake of good drama, they needed one last all out slugfest between Jack & Flock!):rolleyes:

Actually, I was hoping for an instant that the old John Locke would re-emerge. We know that Smoky has Locke's memories along with his body.

I've wondered about this as well. All he said about it is that he needed the body brought back and that he apparently went through quite a bit to make it happen. But he didn't use the physical body. Yet, the body he had was obviously a physical form complete with the ability to bleed and die. This may just be, like God's Tom said, the need for a physical fight, but I'd like to think there's more to it than that.