I pretty much agree with Russ on all counts. Good ol' tap water is safe in many areas, you can check water quality
for yourself if you live in the U.S, and if it's safe for humans to drink I highly doubt it'll kill our plants. Also, rain barrel water amounts do vary by area - trust me I've lived in both New Mexico and Oregon and the difference is HUGE.
I do believe in harvesting rain water for cleaning, plants, and so on, BUT your article is full of paranoia type banter devised to get people on board with rain barrels vs. using real benefits like cost savings and basic conservation. When people get too scared of H2O they do things like buy bottled water and water filters that are hard, if not impossible to recycle, both of which are a drain on the earth. Some people should look for alternative water because their water is truly problematic, but many people have adequate to safe drinking water.