I think the others are a clean up crew
To begin bluntly, I believe that there is some force (this is said in the most general way...ie it could be fate, or an evil cooperation, dharma, government conspiracy, or on man...etc.) that has already decided the fate of those aboard flight 815 before the flight. By this, some plan was enacted to get all these certain people on the plane. This can't be too hard to believe, seeing how many of the fine fourteen were given their tickets (Sayid, Claire, Locke, Kate, Walt, Mike, Sawyer, Jin, Sun). Becasue of this, however, the enitre manifest was not needed... creating the liability of extra passangers.
Those chosen are placed in seats in the mid-portion of the fuselage. The plane flyies to its destination, the tail breaks off as planned eliminating half of the "extra passengers". The remaining portion of the airplane splits again killing those in the cockpit, and leaving the "chosen" on the beach. The original survivors are miraculously unharmed. And thus the true purpose begins be it final confrontation bewteen good and evil or a human expirement.
NOW imagine if this setup didn't go exactly as planned... Imagine if several people got into the "safe" plane section that you didn't intend upon, or even tried to stop from getting on. A rich brother and sister who ended up leaving earier than expected. A husky lottory winner that somehow ended up getting to the plane (maybe despite your best efforts to stop him). A doctor who insisted upon getting home... Even a junkie rocker who had a fix ran from the back of the plane, and ended up, by sheer dumb luck, grabbing a safe seat. Or...even a baby in a mothers womb you didn't account for.
ADDITIONALY the well crafted plan hits another snag when their are people who surivived from the tail section. And a pilot that survives in the cockpit! What do you do? You clean up.
1. Get rid of the taillies...ALL OF THEM. Its pretty obvious that the tallies got whipped hard from the get go...and their had to be a reason for that. None of them were suposed to survive, even the ones who did, ie Ana Lucia (Goodwin intended to kill), Cindy (was abducted), Eko (was attacked early on)...The only one that doesn't fit is Libby (this iim sure makes a lot of people happy...bc it gives creedance thatshe's an other, but im not sure).... And of course good old Bernard, who ...never was supposed to be in the tail section anyway.
2. Get rid of the pilot. Sick your big dog monster on him.
3. Get rid of those who weren't supposed to be in the center section. Hang Charlie, Kidnap Aaron... Drown Joanna? Kill Scott or Steve? Even somehow instigate the death of Boone...or Shannon?
Walt is my only wild card here (along with the questions of who's is doing all this, and why... I'm giving you a reasonable HOW).... they might not have expected Walt to go with mike, or that walt had "talents"
Thoughts?
Remember guys im not trying to answer all the big questions here...and i think this theory fits well with pretty much any theory...so be gentle.
**edits in light of response. +I am not saying specific individual seats are safe... when I say a "safe seat" im refferencing the center-section. And i agree even then its risky... Mars might have been ment to live. I could be that they engineered the center portion to survive the impact... its just as plausible as "nano-techology" or electromagnetism pulling the plane down. I don't even like saying that bc im a theorist who tried to pull ONLY from the facts of the show... anything I don't know I've left as vague as possible... hence "force"