I agree with you about Juliet's motives. She is only working for Ben to get herself off of the island. Jack is simple minded enough to think that desire makes her one of them, when in reality it makes her extremely dangerous. Juliet is one cold fish, and she has one agenda in mind.......get back to her sister. We saw how dangerous that line of thinking made Michael and what he was willing to do to get back to his son. Since at this time Ben is the only one that can give her what she wants, Juliet will be completely loyal to him. However, if she ever feels that he has lost the ability to give her what she wants she will kill him.
Sawyer and Sayid will never be fooled by Juliet. Sawyer looked in her eye and knew she would not hesitate to shoot Kate in cold blood. He will not forget that. Sayid can sense that this woman is the enemy in every way, and will not dismiss that.