Comparing stomach acids to acid rain is ludicrous. (I drink a cup of acid every morning at breakfast, too)
Do you have any idea what makes up the pollutants in air pollution? Of course not. Yes, "clean" is relative. And the relative difference between acid rain and purified water is worth respecting. Do you really want to drink the output of obsolete coal fired plants, refineries, and motor vehicles?
And there are worse contaminants that can be found in ground water and streams than animal feces.
We should pay attention to where our water comes from and what is in it. Municipal supplies are required to report to consumers their regular testing and results. Ever read yours? Know what your system tests for? What is looked for by testing varies from place to place. Some give themselves high grades but test for a fraction of what other locales test for. When I travel to new places I often ask locals where their water comes from. Almost no one ever knows. Yes it is easy to feel all is well when one is not looking, let alone, watching.
The idea of catching rain water for free has deep roots in cruising lore. But what was good for the Hiscock's in the Pacific 70 yrs ago is of questionable relevance today. Especially within a thousand miles of a heavily populated coast. I believe rainwater should be filtered, preferably on input to tanks, but certainly after if not done before. And with a filter designed for industrial pollutants. YOMV