Very few things in this world are necessary, and necessary/not necessary is not the benchmark by which we determine what is best for the population. It's an objective, demonstrable fact that water flouridation is effective in reducing tooth decay in the population to a statistically significant degree. We've been doing this for a long time, and so far the empirical evidence supports flouridation as a safe and effective method for reducing tooth decay, especially in children. 

Most of the negative consequences related to flouride are based on exponentially larger quantities than are present in flouridated water. And injury stemming from water flouridation in western nations is virtually unheard of, and certainly isn't happening in statistically significant numbers.

I believe in evidence-based science, not conjecture, and ideologically driven opinion. As soon as we have an evidence-based, peer-reviewed basis for rejecting the current scientific consensus, I'll gladly adopt the new position. But until then, I'm sticking with the scientific consensus.