10. We simply have to see him again. He can't have been shown every year only for him to make absolutely no appearance in the final season and his mystery to be rendered meaningless. Flocke is stuck now in his Flocke form of course. So Christian will just dissappear? I don't believe that.
9. Christian appeared immediately after the crash where he sent Vincent to find Jack. Why appear as Christian if you're MIB? Nobody is looking at you.
8. Claire said in reference to how she knew the Other's had her baby, "first my father told me, then my friend told me." Clearly she thinks Christian & Flocke are different people. But she knows Flocke isn't John. Why wouldn't he just tell her he was both people?
7. Why prolong the mystery? Claire's comments reopen the question of who Christian has been when it seemed clear he was MIB. But there's no drama now in continuing the mystery if the reveal is just going to be another confirmation that "Yes, he was MIB." There's no suspense in that at all. The only true suspense would come if we were wrong about that assumption.
6. There was never a body found for Christian. Obviously there was one for Locke when MIB took his image. That difference can't be meaningless.
5. Christian identified himself by name to Locke, Frank, & Sun. Why do that if you're MIB? None of those characters know you or care what your name is, and Locke's later reference to Jack that he'd met Christian could have been accomplished regardless of what he looked like. It didn't help MIB for Christian to identify himself as such to Frank & Sun at the very least.
4. It was Christian's influence over Claire that led to the removal of Aaron from the island. Coupled with Jacob's influence over Jack that led everybody off the island and into the past to protect them, Christian's actions here seem contradictory to MIB's goals.
3. Since it was Jacob's influence that led the losties to the past, Jacob would have knowledge of Jin being there. So did Christian when he informed Sun as to when her husband was. But the whole plan would seem mute if MIB had so much knowledge over Jacob's seemingly secret plan. Don't forget MIB was suprised when he heard, "They are coming." He didn't know, but Christian did.
2. It's long been theorized that Christian may have had something to do with the island in the past. Consider Jack's increased importance if this is true. Consider the twist if Christian was the ghost in the chair of the Cabin. Consider if it was Christian who in an effort to get back to the island, could only do so by dying and leading his son there with him, prompting him to drink. A lot of blanks to fill in there, but man that would be some good stuff. Conversely consider why they're even leaving the question open still if the only answer will be to confirm that yes he was MIB. The only character who would care about that reveal at all would be Jack, and he'd be more confused than anything else. He would certainly care if Christian were himself however.
1. He has to have a payoff with Jack. Locke has seen Christian, Claire has seen Christian, Sun & Frank have seen Christian. Hurley may have seen him. There's no way that Christian has been floating around on this island for 5 season's carrying so many unresolved issues for his son if there was never going to be a face to face meeting between the two. Jack's lack of closure with his father has been key to his character, and there has to be a confrontation to resolve that. In fact I would say that issue will be key to the plot, which may mean that Christian has been more important than we gave him credit for.