Filtering water is a good idea because in addition to herbicides, pesticides and such, there is the issue of pharmaceuticals like Prozac, antibiotics, birth control, etc. finding their way into pubic water supplies.
Prozac was recently discovered in Denton streams and therefore in the food chain. Scientists can only speculate on what the full ramifications of this will be.
http://www.google.com/search?hl=en&ie=U ... +bluegills
I personally wouldn't drink any unfiltered water from a public source or a well these days. There's just too much toxicity in the environment.