@dave + @ejeffrey
It indeed seems clear that fluoride is beneficial to dental health for adults, in limiting the occurence of cavities. No discussion about it.
But as Dave pointed out, one implementation problem is about controlling the dose you get out of your tap. An other unknown is how much people use it and how. For adults, this puts efficacy in question. For children with not-yet-mature teeth, well, that raises the problems of potential fluorosis (I have one, sure, not cavities but my teeth are brownish, some of them degraded mechanically when I was a kid - think spitting out bits of teeth).
The thing is that there are too many variables not under control IMHO to safely promote this.
There are so many things that can happen: the minimal dose from tap water can combine with other chemicals (i.e. with Al, forming a compound that crosses the blodd/brain barrier and cause damages similar to Alzheimer) , can raise in quantity unexpectedly (as Dave said, controls are not always made where they would make most sense) and cause fluorosis on their own or combine with other absorbed fluoride amounts and cause fluorosis.
Also, it _is_ common practice to use hexafluorosilicic acid instead of pharmaceutical-grade sodium fluorate. That happens in France where I live in most water treatment factories. As OP said, this is a byproduct of some industrial processes, should be considered (and is in fact categorized like so) a toxic waste as it is contaminated with arsenic, lead, cadmium, heavy metals, phosphorous fertilizers, Al when it comes from the Al industry, etc. Side effects of some of these are well known, some other are not. The studies you are talking about are probably not taking those points into account.
Bear in mind that heavy metals, as well a fluoride have a cumulative effect on the human organism as they are stored and never (or in very small quantity) release by your body. No study can take that into account, same as no study can give you any idea of hormonal disruptions, effects of mixing fluorite with other agents, etc., are those topics are simply not the subject of the studies used to claim usefulness of adding fluoride in tap water.
And as Dave said, fluoride is efficient in local application, not when ingested.
Lastly, remember that Sweden and Germany outlawed adding fluoride to tap water and that most EU countries as well reject that practice. There must be a reason ?
Forced medication is not something ethically sound in any way, I just find it disgusting and morally wrong. People who don't should at least respect the
wish of those who do not want to be forced to ingest any kind of medecine without them contolling it. Dammit, just brush your teeth and you'll be fine - as long as you do not swallow the toothpaste, which in addition to fluoride often also contains endocrinian perturbators like triclosan, which are too bio-accumulative, and that can also be found in tap water, but that's a story for an other day ;-)
There is enough crap in our water due to human industry, no need to voluntarily add some more - unless you like russian roulette.
Edit:
"It can cause discoloration and increased brittleness, but overall it is a clear win for dental health."
This is your opinion. Statistics might show a global improvement of dental health, but statistics tell you nothing about individuals. And as an individual, I'd rather care for my own teeth myself and reduce my risks of fluorosis (too late for me, take that in a past tense) that cause the brittleness and discoloration you are talking about.