After this, I had to wonder why Hurley only saw some ghosts, but not others. Like, on the island he should've spotted tons of folks. If it's only folks who knew him or want to send him a message, I suppose that works with Dave, since Dave wanted to talk to his crazy wife Libby, who was also in the institution.
Not the same guy. Dave was imaginary; Libby was married to another guy named David - this has been verified by Damon Lindelof.