IF the info is available and you can study it on your own, why sign up? You do not get college credit for it, you cant put it on your resume.
Because having exams at a set time is a great motivator! I've had the AI Modern approach book sitting on my book shelf for over a year until now.
with 25,000 registered you have zero chance of asking the prof a question.
Well the prof is only going to answer the top rated questions. So if other users find your question useful and it gets rated up, then there's a good chance that he will answer it.