Let me first start off by saying that I do not want to make this into one of the many "where's Desi? whenz he comin bakk? oh noes!" blogs I've seen across the Internet, I do believe that having the character who is still ranked as the fan favorite off-screen for so many episodes is not a very good idea.
Unless.... they are building up to a big return.
Now why would Desmond return to the Island? Some believe it's because Ben killed Penelope. Really? How can anyone think of such a thing and not be morbidly saddened? They have a son... do you really think the writers would allow Penelope, the woman who our Romeo has been tracking down for the past four seasons, to be killed off simply for plot convenience? Probably so, but its still horrible. The killing of Nadia is one thing; she was almost never on-screen, and Sayid dropped the notion of her temporarily when he met Shannon anyway. The killing of Penelope would be tragic. And a poor choice on the producers' part.
I'm of the belief that Desmond is the true hero of LOST. Everyone's looked to Sawyer, Jack, and Locke before as the main savior of all on the Island. But I think it's Mr. Hume who will pop up before the end of the season. I also believe that he will be the one to kill adult Ben (if and when he dies).
[The following is not a spoiler. Just speculation.]
There are some who report that in Season Five's finale, a lead character and a secondary character will be killed off. The concensus among many is that those unfortunate souls are Ben and Penny. Is that the case? And if so, here are a few questions: (1) Who will be the antagonist for our Losties after Ben's gone? (2) What will possibly be the motivation for Desmond if his literally life-long love is dead?
Your thoughts and comments about the show's greatest character are more than welcome.