I think it's really important to consider the repercussions of the actions in the FST and whether they will impact what is happening currently on the island. I say this mainly in terms of life and death. We know that although characters in the OT may be dead they can still exist in the FST (although are seemingly aware of this alt. world status). However, if a character dies in the FST, what will happen to them on the island?
I ask this because I believe Desmond is trying to kill Locke in the FST. Obviously, him running him over in the car implies this. Originally I thought it was just his way of forcing connections amongst them - ie Jack having to do Locke's surgery, Locke and Sun being wheeled into the ER at the same time. If Locke dies in the FST, either by Jack botching the surgery (accidentally or on purpose), what will happen to his body on the island? Will MiB no longer be able to use his form? I think this is the case, and that is why Desmond believes he needs to be dead in the FST. Perhaps if Locke dies in the FST, MiB will only be able to run rampant as the monster again. Illana did say that he would be stuck in this form from now on, now that Jacob is dead.