Mineral water was in fashion a decade ago, today De-mineralized RO water is in fashion. I can understand the need of people who live near the sea, asking for desalinated water for drinking, but why the rest of the country is going for bottled water cans and RO purifier.
There is no single WHO report that corroborates that drinking hard water has any adverse health impact (as long as you are not drinking sea water or equivalent which is manifold higher than what most borewell or piped water has). On the contrary there is a WHO mandated minimum salt (TDS requirement) that any drinking water purifier has to maintain. Even the web literature on hard water talks about its impact on soap, boilers etc. There is no impact of dissolved calcium & Magnesium on the bones, kidney stones etc. In fact, human beings need calcium either through diet or supplements to maintain their bone density.
The claims of RO treated water has lesser microbes & dissolved organic matter is also without basis. RO is designed for controlling the amount of dissolved salts only, it relies on an inbuilt traditional water purifier/filter to decontaminate the pipe water before feeding it to RO. In fact, 100% salt removal adversely impacts the taste. So the so called TDS controller actually only blends the normal (purified) water with the desalinated water to achieve desired taste/salt content. Point being if traditional purifier were so bad in decontaminating water, the whole principle of TDS controller would fail.
Domestic RO machines are expensive units costing 5-10k each and every 6-8 months the membrane needs to be replaced which costs almost ½ the unit cost itself. Even after ignoring the cost of piped water & electricity, the personal RO water cost almost the same as the commercial bottled water. Also to generate 1 glass of RO water, the filter wastes away another 3 glasses of piped water through the thin white pipe from the purifier to the drain. In a country, where ground water levels are depleting and portable water not available to 80% of the households, this seems like an expensive luxury.
There are some regions which have dissolved heavy metals, nitrates and other dissolved contaminants for which RO is the only treatment mechanism. However just using a blanket RO filtration everywhere may be foolhardy. A simple 100-300/- water purity test can prove to you that the RO marketing campaign is nothing more than a gimmick to make you spend more.