Question

What is the advantages and disadvantages of using sodium carbonate to soften water?

Answer 1

The advantage is that most of the Mg and Ca ions are removed which means less detergent needed to clean. It also means less "bathtub" ring when the Ca and Mg ions react with soap. The con is that the Ca and Mg ions are replaced with Na ions and most health agencies advise against consumption of too much Na due to high blood pressure and others. You can read about this here but I advise caution in reading this since much of the material is biased, in my opinion.
http://www.aquatechnology.net/softenerproandcon.html