We know this material is toxic... so why has it not been extensively tested for human consumption?
Fluorosilicic acid is a substance most commonly derived from the pollution scrubbing devices of the superphosphate fertilizer production industry. It is pathogenic to plants so has to be removed as part of the production process... no sense having fertilizer that kills your veggies, after all. It is a classified hazardous waste, is highly toxic, and yet manages to be the substance used in 90% of the water fluoridation programs in the United States (and presumably elsewhere as well) This fact has raised concern amongst health risk assessment scientists at the EPA who have helped draw attention to the fact that the only other place this fluorosilicic acid can legally be disposed of is in a hazardous waste facility. As Dr. William Hirzy, Senior Vice President of the EPA's Professionals Headquarters Union, put it:
If this stuff gets out into the air, it’s a pollutant; if it gets into the river, it’s a pollutant; if it gets into the lake, it’s a pollutant; but if it goes right straight into your drinking water system, it’s not a pollutant. That’s amazing! 
Note also that this union, comprising mostly scientists and engineers at the EPA, is strongly against fluoridating water supplies… and who would know better than them regarding this issue?
Wow. That sounds bad. Too much so to be truly believable, frankly. So I did some more study, now on fluorosilicic acid, which is what seems to be in most of the fluoridated water.
One of the very first things I discovered was that there has been virtually no testing done with regards to the toxicity of fluorosilicic acid in municipal water. I came across a "Sodium Hexafluorosilicate and Fluorosilicic Acid: Review of Toxicological Literature" dated 2001, and in the opening statements I found this:
Sodium hexafluorosilicate and fluorosilicic acid were nominated for toxicological testing based on their widespread use in water fluoridation and concerns that if they are not completely dissociated to silica and fluoride in water that persons drinking fluoridated water may be exposed to compounds that have not been thoroughly tested for toxicity.
The article goes on to list the uses of this waste product as follows:
The major use of sodium hexafluorosilicate and fluorosilicic acid is as fluoridation agents for drinking water. Sodium hexafluorosilicate has also been used for caries control as part of a silicophosphate cement, an acidic gel in combination with monocalcium phosphate monohydrate, and a two-solution fluoride mouth rinse.
Both chemicals are also used as a chemical intermediate (raw material) for aluminum trifluoride, cryolite (Na3AlF6), silicon tetrafluoride, and other fluorosilicates and have found applications in commercial laundry. Other applications for sodium hexafluorosilicate include its use in enamels/enamel frits for china and porcelain, in opalescent glass, metallurgy (aluminum and beryllium), glue, ore flotation, leather and wood preservatives, and in insecticides and rodenticides. It has been used in the manufacture of pure silicon, as a gelling agent in the production of molded latex foam, and as a fluorinating agent in organic synthesis to convert organodichlorophosphorus compounds to the corresponding organodifluorophosphorus compound.
In veterinary practice, external application of sodium hexafluorosilicate combats lice and mosquitoes on cattle, sheep, swine, and poultry, and oral administration combats roundworms and possibly whipworms in swine and prevents dental caries in rats. Apparently, all pesticidal products had their registrations cancelled or they were discontinued by the early 1990s.Fluorosilicic acid is used in the tanning of animal hides and skins, in ceramics and glass, in technical paints, in oil well acidizing, in the manufacture of hydrogen fluoride, for the sterilization of equipment (e.g., in brewing and bottling establishments and for copper and brass vehicles), and in electroplating. It is also employed as an impregnating ingredient to preserve wood and harden masonry and for the removal of mold as well as rust and stain in textiles. (note: PDF link)
The article goes in far greater detail on various issues... it's worth reading if you want an objective overview of a request for toxicological screening (which, as far as I am aware, has still not happened.)
So there you have it: It's mostly for you and me to drink, but also useful for killing bugs and rodents, tanning hides, and general usefulness in industrial applications... AND YET IT HAS NEVER BEEN SERIOUSLY TESTED FOR HUMAN CONSUMPTION/TOXICOLOGY!!