I hear your discussion regarding the collection of rain water and would like to add that it is important to use a system that does not facilitate an explosion of mosquitoes. Mosquitoes are vectors which carry a number of diseases and it is important to control the population. Pls check out the Facebook page for Marin/Sonoma Mosquito & Vector Control District for examples of good and bad methods for collecting rainwater. The good methods make it difficult for mosquitoes to breed.
You make a valid point. Standing water is an instant breeding ground for mosquitoes. Even a small amount of water, like an overturned soft drink cap, can collect enough water for a mosquito to lay eggs.
Most rainwater collection systems are covered, with the home's downspouts depositing roof runoff directly into the barrel, leaving no exposed standing water. The rain barrels often have a spout or spigot to which you can attach a garden hose to when it's time to use the water for your grass, plants or garden.
More worrisome than a rain barrel are the objects that often collect in a home's backyard, like pool and sand toys. These items can collect rainwater as well... and if not emptied regularly, can become a breeding ground for disease-carrying mosquitoes.