Only downside with RO is it uses more water, but depending on water quality and cost it is often wirth it. Over on the carbon filter side there is KX media, which does a redox reaction and can kill giardiasis and help with slight contamination. It also does a great job against chlorine but chloramine is harder to filter.
If you live in my corner of the USofA the concept of "wasting" water is meaningless. I actually posed this question to an LA water conservation engineer guy. He didn't know how you could possibly water here.
