We have a water softener on our house system and the company we got the system from is the same company I rent my large DI tanks from (culligan in the US). Our system is a typical system in that how it works is by replacing the minerals that make water hard with sodium. So if our TDS is 200 before the system it is still 200 after the system only instead of calcium or magnesium, it is now salt. The bennifets are not-so-much for the DI unit, it would reduce mebrane wear big time, but now our house has soft water which reduces hard water buildup in all of our home water issues. Laundry, dishwasher etc. it also reduces the amount of soap needed by half. It also makes taking a bath more of a pleasure. Clothes and skin and everythng just feels softer...hence the name I suppose.
If you wantd to extend the lif of your membrane you should put a carbon filter (as big of one os you can afford) in front of the RO unit which will reduce a lot of the stuff that kills your membrane. Here in the US carbon is cheap compared to mebranes. It is suggested that you use 50 to 60 pounds of carbon but even less if replaced as needed would suffice to seriouls ad life to your membrane. Also, make sure you have the ability to flush your membrane often.