Belief systems in and of themselves may not constitute knowledge, but they are definitely something that one can be knowledgeable about. Given the number of people who subscribe to some of the most popular belief systems, and the fact that one cannot go far without interacting with a person who subscribes to one of these systems, it is genuinely useful to know about how these systems work, how they are similar, how they are different, and what they are really all about.
If people were more informed about religion, both their own and others, maybe the level of discourse after 9/11 would have been better than "Kill the Muslims, they hate us for our freedom!"
Of course, there is a difference between teaching religion and teaching about religion, and it is difficult for some people to teach this kind of thing evenhandedly. Small changes in language could set people off, e.g., "Christians believe that Jesus was the son of God, sent to give us salvation" vs. "Jesus was the son of God, sent to give us salvation". And then there is the issue that, if someone has strong religious beliefs one way, they will not take kindly to the perception that their children are being indoctrinated in another belief system.
If astrology was a widely held belief that affected the way laws were passed and who we went to war with, if people were apt to get into big arguments over astrology, and if a working knowledge of astrology was required for someone to understand many of the big arguments in our society, I would advocate that they teach a little bit about astrology in school too. Just make sure to teach it in a way that says "this is what astrology is" and not "this is what is true about the world." The danger is not in explaining a religion, it is in espousing one.