This theory is purely about the structure of the show and how we perceive the importance or lack there of, of certain characters.
You see, what I see when I look at in a show like this, is that the writers have developed a highly detailed show that leaves only a little flexibility in the storyline. It's not like Heroes or 24, where a storyline last for a season, and then they can carry on after that in any way they choose. From the beginning, Lost, has had a specific end that these writers were always aiming for.
But as with any show, it had to be adapted. It's success meant that the writers had to find creative ways to "stretch" the storyline out over six seaons, instead of the 2 or 3 they had initially planned for. And the stretching of the show is what I'm talking about.
Take Desmond for a example. A character with massive potential through season 2, who had a big season 3 with his influence over Charlie and all that has led to, and then who through Seasons 4, and esspecially in Season 5, has dwindled off in importance.
No doubt that Desmond has a role yet to play in the finale season, but it would seem at this point, that if you made a dot on a rubber band, to signify his importance, that when you stretched it out, that dot ended up almost completely on one side.
What I'm trying to say is that we perceive that Desmond has this huge roll left, and we do that because he was presented as a game changing type of character. But I would argue that he already has been and that's all there is for him.
Just imagine for a moment that the show had been just 3 seasons. We would have found Desmond in season 2, he would have come back, pushed the button, been involved in Charlie's death, and the island's discovery, and at that point the show would have been ramping up for it's conclusion. In other words, Desmond's importance would have parralleled the show's climax. He would have done exactly what we've seen him do, but since the show would have ended soon after, your impression of his importance to the story would likely have been much higher. As it is, we're all sort of wondering what in the hell he's doing.
Sawyer on the other hand, had a dot on the rubberband that was stretched nice and evenly. His character has steadily built, slowly becoming more and more important. In fact, had the show been concluded after 3 seasons, I don't know that we would have such a likeable character now. The writers were able to use the extra time to build him up as a genuine hero rather than a pure anti-hero.
A character like Michael had a dot that was left in the beginning of the show, but unlike Desmond, the writers had never intended, even in a shorter version of the show, to have him play a significant roll at the conclusion, and so they had every reason to kill him off.
Claire in a much different way, had her dot on the rubberband seperated evenly in two. She played a nice roll very early on, had slight bits through season 2, and then had been somewhat non-existance thereafter, even literally in season 5. But it's fairly clear that she'll have a very significant part to play in season 6. The writers simply didn't have a need for her in the middle of the show, and you might really wonder how her character would have appeared differently if spread out over only 3 seasons.
The moral of this theory is that, you kind of have to guage what the writers intent for a character was, and balance that with how you've been forced to perceive a character due to the length of the show. And I'd do that to avoid assuming that character A is going to play some vital roll, when in fact they already have, and you just think they will do more because they're still around. Just imagine the show as having taken place over 3 years instead of 6, and think about how significant any one character would be in that situation, or if they would need to do any more to justify their significance between now and the end of the show. Some of them would seem quite different.