The net effect can't be that big...
The amount of water a typical home drinks and thus sends through a filtration system is a tiny percentage of overall use. How much water does a typical person use for drinking and cooking each day? I'd be surprised if it's even five gallons. When compared to the water used for toilets, bathing, and washing clothes, it's a tiny percentage (outside water used for watering -- the bulk of most home use -- mostly stays in the ground though there's still runoff that winds up at the water treatment plant).
And RO doesn't add any pollutants that weren't there already. The water a RO filtration system sends down the sewage line is a little more concentrated than before filtration, but it gets diluted at the water treatment plant. The alternative is that people consume unfiltered water with all the trace contaminants. Either they absorb contaminants (e.g., lead) or they excrete them and they get passed to the water treatment plant anyway. I guess I'm selfish, but I do care about this stuff getting into my body especially when it will be in the general water supply anyway if I don't treat what I drink. It comes down to either using a RO system to filter contaminants, or essentially using your body.
The big question whether or not all the trace contaminants such as lead, arsenic, urine salts, soaps and detergents, excreted human medications, fertilizers and pesticides accumulated from farmland runoff, etc. are a factor. Clearly they're not on a day-to-day basis, but the unknown question is how much they affect us over a lifetime.
The final evidence is not in about the long term effects of trace contaminants, but I don't think anybody should feel guilty about the affects on the general water supply of installing a RO system.
