I grew up in US public schools in Missouri and Nebraska. My family was Catholic and sent me to Sunday school. I learned absolutely nothing about other world faiths until I elected to take a course on comparative religions at college. It was only a basic introduction but it Blew My Mind. I felt utterly conned that up until that point in my life, no one had seemingly wanted me to find out about what other people believe. WHY? This is a good question for this thread maybe. As I learned, I came to decide that the world view I had been taught did not match reality. I went from someone who wanted to become a nun as a child, to what I'd best describe today as an agnostic. With leanings toward Buddhism maybe. But no one was about to teach me what that is or what anyone could learn from it. I suspect this is because, as has been suggested here, people think that religious education will cause the "faithful" to jump ship. I would argue that's not necessarily a bad thing, but plenty of people would equate it with condemnation.
Perhaps because of my own personal experience, I do believe that a broad education broadens the mind. Concealing the truth from students, e.g. that there are other religious views in the world, and different ways of looking at things, is antithetical to what any good teacher should be doing.
RE seems to work well here in the UK. Indeed it is necessary. Leicester, where I live, has an Asian population of over 20%. Learning about Hindus, Seikhs, and Muslims is part of learning about the people who live and work here, those with whom we interact. In fact, RE can be a way for them to learn about the Christians here too. I taught for a while at a school that was 95% Muslim and I am sure that without RE, quite a number of those students would have known little to nothing about Christianity.
The big question would be how to introduce this in the US though, where religion is so much more of a political issue. I don't know how it could be done; all I can imagine is problems and lawsuits.