I think that one of the underlying intentions of Orwell, despite the harshness of the overall tone of the novel, is that he intends to assert that there are 'higher' or 'deeper' instincts and drives in the human spirit. Even though Julia is seemingly a product and a victim, her spirit drives her to somehow maintain something genuinely human and sensitive, even if it is her 'sub' or unconscious response to do what she does. She is to the Party, subversive, in that she is also a member of one of the Party's tools that strives to supress anything that is genuinely human and sensitive, as it poses a threat to their power. By sleeping with party members, she also serves to illustrate the falsity and prententiousness of those who try to control others for their own sake of maintaining power and control over the masses while not 'practicing what they preach'. Her liasons with Winston are not just for sexual contact. They talk, listen, share food, and so on. One may say that possibly Orwell shows that there is a 'chemistry' between the two that they were drawn to because of some remnant or intuitve sense of 'goodness' that is still there, even though they will never recognize it as such, amidst the brutally controlled and maintained political and social (so called) order.
I think that part of what makes Julia so appealing to Winston is that she is his countervailing force. While he is obsessed with undermining the party and political expressions of the good, she is not. When he reads Goldstein to her, she falls asleep. Whereas Winston is at first consumed with his complete disdain for the party, she expresses herself in private as someone who does what she feels. It seems a bit as if he is weight and she is lightness, to borrow a Kundera bromide. The fact that she enjoys sexual contact is something that is also appealing to Winston. She enjoys it for its sensual value, while Winston enjoys it for its political statement. Julia is a product of the system around her in that she works for the party as it is socially expected of her. As being a part of the system, she undermines it with her private expressions. In some respect, this is what also makes her so attractive to Winston, in that she has been able to construct a private and public facade, something that he has not been able to do.