+1 with what the Geek said. There are places all over the world that have near rainwater quality (or it is rainwater) and they are doing fine. Drinking low mineralized water is not a health risk unless your diet is also deficient in minerals. Since the bulk of our mineral intake is via our food, the quality of the water is a minor factor.
I had Jim's concerns several years ago when I first considered a RO system, but after more thought and research I found that my concerns were unfounded.