a lot of us think that the flashsideways is actually created by whatever happens in the finale. we first came to this conclusion trying to explain why jack has that mysterious cut. i'm thinking that desmond and MIB have some kind of encounter, which destroys the island, leaving it on the bottom of the ocean. it might be possible that the MIB is now inhabiting desmond (maybe sharing his consciousness, not unlike "being john malkovich") in the flashsideways and that's why he's so eager to get everyone together. maybe this flashsideways was the something "beautiful" locke saw in season 1. the flashsideways also does match up with the promise MIB made syaid in regards to seeing nadia again.
perhaps the real reason that desmond wanted locke to "let go" is so locke will have his legs back and then the MIB can shift to locke's body and jump back into the original timeline as flocke, thereby giving himself the chance to destroy the island once and for all. desmond has proven that he can jump timlines in "before your eyes," so maybe the MIB can use that to his advantage.
i know there's a lot of holes in this theory, as if it follows "bill and ted" logic... but it's the last stab at a crackpot theory i'll ever get to take at "lost" so i'm okay with how silly it is.