Saltwater corrodes firefighting equipment and may harm ecosystems, especially those like the chaparral shrublands around Los Angeles that aren’t normally exposed to seawater. Gardeners know that small amounts of salt – added, say, as fertilizer – does not harm plants, but excessive salts can stress and kill plants.
I would imagine the thousands of cubic kilometers of freshwater currently entering the ocean from global warming far outbalances the little water we take from desalinization, and the net effect even if we put that salt back is quite a bit lower salinity.