Well, my biggest thing is 'is it necessary today'. I would argue it is much less necessary to have fluoride in the water today than it was then they started. I've had only one cavity in my life, and otherwise have had perfect dental health - this is not due to fluoride, as my home has a well to pump our drinking water from. As I've stated before, dental health has come a long way - toothpaste is much improved, for instance. So, the question is, with today's tools for dental health that are readily available, are the risks brought about by fluoridation still outweighed by the benefits?
I have a feeling it's yet another case of what I like to call "bureaucratic inertia": something is in place, and it would take a lot of effort to remove it, and it's likely that your popularity would go down if you did because you'd have to explain in great detail why you're doing what you're doing and it's inevitable that
somebody wouldn't like it, so you just don't do anything about it.