What many people do not realize about fluoride is that it often occurs naturally in wells that are used to obtain drinking water. Some wells have none, some have the recommended amount according to the ADA, and some have excessive amounts that can cause staining and improper tooth development called fluorosis. So when you hear people say that water fluoridation is a poisoning of the water supply with a "chemical," remember that this is a very uniformed statement. Everything is a "chemical" and is dangerous when not used in moderation, such as vitamins, oxygen, water etc. The reason for public water fluoridation is two fold: first, the benefits to dental health with proper fluoride intake is staggering and arguably the biggest advancement in dentistry; second, by managing fluoride levels the benefits can be realized without the risk of getting too much or too little.
With the popularity of bottled water, which often does not contain fluoride, more and more people are not benefiting from its tooth stengthening ability.
Facts About Fluoride: http://www.ada.org/3088.aspx?currentTab=1