I get the sense Juliet is only looking out for herself and essentially doing whatever she thinks will work to get off the island. At first, it was using Jack to kill Ben. After that, it was agreeing to get Sawyer and Kate off the "Alcatraz" island so Jack would do the surgery on Ben. Now, it's working with Ben to do whatever it is Ben wants with the Losties; apparently, she will help set the Losties up for some sort of invasion from the Others?
One has to wonder why Juliet (or anyone) believes anything Ben says or agrees to - he is a self-confessed manipulator. Suuuuure, Jacob healed Rachel but where was the proof? The information in the medical folder was blatantly fake (the sex was listed as Male). All he showed her was Rachel playing with her kid - admittedly, an emotionally trying scene for Juliet anyway. Perhaps there's a double-double-double cross in the mix and Juliet will ally with the Losties. Since the viewers' sympathies are with the Losties and specifically Jack, we have to believe him when he says that he saw in Juliet's eyes the desire to get off the island. It's just his trust that might be betrayed by Juliet's overriding agenda.
One of the things that I find interesting about LOST is that they continually refuse to categorize people into "good" and "bad" - a very common trait in the American mindset. Look how many people commenting on boards and podcasts are now calling Juliet a stone-cold we-yotch. I think one of the themes of LOST is that people are complicated, they are many shades of grey and take actions for many different reasons and you shouldn't try to just categorize them into two pools. I wonder how much of this outlook is lost on the quick-to-judge, "there's us and there's them" American.
Anyway, I think LOST has really been cooking recently. If you think of it as a novel, then we're about 50%-60% of the way through the story. This means it's just about time for everything to move up another notch. Sounds to me like the poo hasn't even hit the fan yet.