OK, so they are all there to heal past wounds. That's it and I'm convinced. Those that stay either are healed, don't need to be healed or are still working on the process. That whole thing with Locke and his nutty dad showing up at the end tied up is so that Locke can say all he has to say, do what he needs to do an get out all the pain and hurt that has built up over the years form all the bad things his father did. Once that's done, Locke is done with his "REHABILITATION" on the island. As for the other islanders the same goes for them. From the second they crash landed there the healing process began. Take Mr. Eko - he did what he had to and now he's gone. Shannon learned how to truely love. The second Locke got on the island he could walk again and that was kinda like a freebe to start the process.

So I'm convinced. If you have a lot of pain and suffering or post truamatic stress disorder caused by some horrific even that needs to be healed go to Healing Island where the Others can help you heal.