if you use it correctly, Fi can be an aid in the INTJs already well developed ability to read people. NFs (especially NFPs) can read people just as well as INTJs, we just do it a little differently. I use Fi to almost psychicly get in tune with the other persons real emotions and desires. Ni can predict what will happen, and Te can predict, logically and objectively, what someone will do. Fi can give you a window into the person's true intentions and beliefs and allows you to see who they are, not just their behaviors.