There are few benefits of having softener and whether one is needed or not totally depends on how important any of those benefits are to them & their family. 1. To protect pipes, appliances ( water heaters from leaks, dish washer etc.) and faucet from damage ( leaks, reduced water flow) due to scales.
Water hardness is defined as amount (mainly calcium) of calcium & magnesium present in water. Generally up to 3 GPG is considered soft water and after that gets classified into low, medium and high hardness depending on the hardness level. Dublin, San Ramon and Pleasanton all come under zone7 water district and generally hardness tend to fluctuate between 15-25 GPG. Mostly being around 19 GPG. 2. Other reasons being symptoms like spots on glasses, dishes, dry / dull skin & hair due to difficulty in rinsing away soap completely, white clothes turning brownish in wash cycles etc.
That said, world health organization says there does not appear to be any convincing evidence that Water hardness causes adverse heath effects in humans. So does, some other prominent organizations. Bottom-line take away is: If water hardness in your area is more than 3 GPG ( which in most parts of Dublin likely to be around 19 GPG), it is your call on if you want to invest in protecting your water lines, appliances and benefit from other things such having soft skin, hair, nearly spotless wash etc.