It depends on the class and the professor. A lot of professors do not review the readings in class...they may use them as a basis of the lecture, or lectures may be entirely unrelated to the readings, or they may be comparative to the readings.
A good number of professors "encourage" class attendance by having some exam questions based on things covered in lecture only, or by asking comparative questions of material covered in the lectures vs. that covered in the reading.
Another point to consider: professors are aware of who is in class, and who is not. When that strange face shows up at office hours, seeking help or seeking a favor? or sends an email seeking help or a favor, and the professor cannot put a face to the name on the email?