I believe dreams, visions, and premonitions exist in the show. This is a completely seperate topic.
What if the "ghosts" are really just time travelers who have been "unstuck" in space and time?
By becoming "unstuck", these individuals get to see and experience the past, present, and future.
Even though Charlie, Christian, Shannon, Boone and Ana Lucia are "dead", they can interact with with the living, giving them guidance.
Like with Quantum Leap, traveling to different points in time give the "dead' insight into what has happened---what mistakes have been made---and what should be done to change the future.
Conversations with these ghosts vary in length and they end with the dead person "disappearing." Much in the same way Jin must have disappeared when he was standing in front of Rousseau, or the way Locke disappeared when speaking with Richard Alpert
I think certain "special" people are more in tune with these travelers---Miles, Ben, Locke, Hurley, and Walt, and therefore have much higher incidents of interaction with them.
What do you think?