The Smoke Monster, Time Cop

In S3E15, "Left Behind" Juliet and Kate encounter the smoke monsters in the jungle.

I believe the Smoke Monster was evaluating Juliet's memories (as it apparently did with Eko in S2?) and determines that she can not die yet because...

She does not yet have the memories that the Smoke Monster has of past events, that is... She has to live in order for the past to happen as it should/has always happened. This could also be a variation on the whole "You have work to do" and "You can go now." The island/Smokey/Jacob know that certain events must happen or have always happened, so they do what they can to make it that way.

Later in S4E9 "The Shape of Things to Come" after Keamy kills Alex, Ben proceeds to apparently summon the Smoke Monster to take out Keamy's team.

Anyway, the Smoke Monster let Keamy live... because if he had died at that point, the Freighter would have exploded much, much sooner. The helicopter wouldn't have taken off, Ben wouldn't have had to turn the wheel, and the "Flashies" wouldn't have become part of the islands past which is (I believe) how it has always happened.

So the Smoke Monster seems to be some kind of Time Cop, or at least aware of who can die and when.

That's all I've got, until next post...