Softened water will assist to extend the useable lifespan of the RO membrane, but membranes have become less expensive so extending the lifespan may not be as critical as it had once been.
Salt (sodium chloride) is most commonly utilized to produce brine to regenerate the resin's hardness removal capacity. During regeneration, sodium from the brine will adhere to the resin beads whereas hardness mineral ions (calcium & magnesium) that had adhered to the resin beads while water was being delivered to the home, will be released and will be flushed to drain along with the chloride from the brine. During Service mode, hardness mineral ions will adhere to the resin beads which will cause sodium ions to be released into the water flowing to the home's faucets & fixtures.
Because the quantity of sodium within softened water is proportional to the amount of hardness being removed, and because RO membrane is so effective in removing sodium, an RO system for drinking water is usually essential when the softener is removing a substantial amount of hardness such as 50 GPG or greater.
Here is a brief writeup regarding the quantity of sodium within softened water. How much sodium is in softened water
Because so little water can pass through a typical undersink RO membrane per minute, without a storage tank, it would require an excessive amount of time to fill even 1 drinking glass.
An 'undersink' RO system does not need to be located under the kitchen sink. Our laundry room is located in the basement directly below our kitchen. For better space utilization and ease of access for RO system maintenance, I located our RO system in the laundry room.
Because of the extended distance, and because our system supplies both the sinkside faucet and refrigerator ice/water dispenser, I utilized 3/8" tubing instead of the more common 1/4" tubing so as to reduce any flow restriction caused by the longer supply line.