Rain water is evaporation same as distilled therefore impurities within the water evaporated would not persist. However, just like distilled water, rain water would be exposed to elements which might pollute such as industrial waste gassed into our atmosphere and some leaching that could occur with distilling machines. Yet both can be purified further with RODI which is currently done with distilled. Point of capturing rain water isn't to bypass filtration such as RODI but utilizing less of it or as a more feasible way of accessing water. For some their rainwater may be serviceable with just sediment, carbon and DI and others might need RO added.Oh yeah, I just reread your posts and you were 1 person who said that! I noticed you also mentioning that you assumed rainwater wpuld be similar to distilled water plus all the pollutants but if he invests in a filter system, it should be fine...Im not sure why you would expect rainwater to be similar to distilled water or why you believe that the pollutants you believe to be present would be filtered out as efficiently as tapwater through an RODI filter, but hokey dokey...
Not sure why you think tap through RODI would be purer than rain water through RODI?