With ozman
Smokey existed before Jacob and his brother arrived.
Smokey played the part of Claudia... seducing Jacob's brother away from Jacob and his mother
We have seen smokey imitate dead people all through the series
The Darkness (smokey) has been wanting to be released since the beginning of time, Jacob's brother just shared the same goal of getting off the island
Hurley seems to be the only one that can see 'real' dead people... note how Jacob could not see Claudia, but his brother could
Just thinkin' out loud 
But Jacob couldn't see Claudia. So it's not like the times we've seen Smokey manifest as a dead person.
Huh? Who saw Christian besides Jack? Locke saw him with Claire and again with the Donkey wheel, but not with anyone else. Who saw Yemi besides Eko? Who saw Dave besides Hurley?... I think that smokey can appear w/o someone else seeing him... other than the target that is. Illana asked Ben who he was talking to when smokey appeared to him while he was digging his grave.
Well, Young MIB could see Claudia because he is "special". Kind of in a way that I'd assume Hurley is "special" in being able to see dead people. As far as Smokey appearing other times...Jack can't see dead people, Eko can't see dead people, and everybody who is seeing UnLocke (except Hurley) can't see dead people.
Also, I could be wrong, but I don't think Illana asked Ben who he was talking to. She just looked over at him. But her not seeing UnLocke at the time can be explained quite simply in that he was behind some bushes.
Finally, I'm not 100% sure Dave was MIB. In fact, my guess is that he wasn't, since we've seen a bunch of times where Hurley was the only one who could see someone.