The producers have said that the FST was an important part of the story of the show, and now we have seen, from The End, that the sideways timeline was basically a class reunion after everyone involved with the Island finally died. Which brings up an important question: HOW IS THAT SIGNIFICANT? Theoretically, you could pretend the same premise for any story. Lord of the Rings? You could pretend that after everyone died they all lived an "afterlife" on a Middle-Earth not corrupted by Sauron. Or that the people in Star Wars all lived in Endor after their deaths. What significance does an afterlife have on the story we all care about? Couldn't we have just watched Jack save The Island and let it be done with?
I don't think so. I still think the sideways timeline is important, and here's why and how:
First, let's ask ourselves one more question: Why did the show present us with this new "timeline" at the start of Season 6? If the "afterlife" has no temporal boundaries, wouldn't these people be building/experiencing it throughout the entire show? Why would the show introduce this new setting (and I think "setting" is the most appropriate term to describe what the FST is now) after the ambiguous H-bomb detonation at the end of Season 5? This is why: The detonation of the hydrogen bomb created the new setting, which is actually not an afterlife.
The Island's magical/special/hyperphysical properties somehow, combined with a hydrogen bomb detonation, created a new setting that took on characteristics that the people involved (i.e., Jack's bomb crew) wanted it to have. Jack et al wanted the detonation to achieve a reset, in which the flight doesn't crash. Yet I suspect that somewhere in the backs of their minds, people knew that they would lose each other. And because, as Christian said, The Island is where they experienced their most meaningful days and relationships, this unspoken desire also got imprinted onto the new setting.
Think about it. At what other point in the show's story did every still-living major character actually agree on a major course of action and participate cooperatively and in emotional union, other than perhaps during The End when they all decide that Smokey needs to die? The shootout in The Incident remains one of my favorite "fellowship" moments because of that factor. The shared purpose everyone felt during that moment might have had something to do with this "imprint" for the new setting. The Source and the Island's overall importance to both the Losties and the world might have had something to do with it too.
We see the new setting for the first time at the moment when the crash originally began because that moment was the moment everyone was focused on changing during the detonation. That moment marked the start of the slow realizations. And because time doesn't work in the new setting the way it does in the normal world (okay, this factor is the one that most supports the idea of the new setting as an "afterlife," although I disagree with calling it that), when Juliet is about to die and can begin to experience the "afterlife," she takes some of her new-setting life to the Island life, telling Sawyer that they could go Dutch.
Based on all the above and perhaps a few other things I haven't worked out into words yet, I think the FST was immensely important. It answers the question of what the bomb detonation was really all about. Without it, Jack and company simply set off a bomb that returned them to their proper decade. I don't think the FST was an afterlife. I think it was, as Desmond has always said, another life.
Of course, this still doesn't answer why the Island would be underwater, unless we also take the "imprint" aspect a bit further and say that the Detonators also didn't want the Island to have anything to do with their new lives, thus rendering the Island a moot point.