Who are these people that add salt to their freshwater tanks on a regular basis? Freshwater fish (unless brackish) do not like salt. Freshwater plants do not like salt. What benefits would this provide? The only times I hear people add salt to their freshwater tanks is as medication to combat stuff like parasites.
Having said that I have no idea about mechanically softened water / if it could be harmful. Out of curiosity are you looking to soften your water in the first place?
I agree with your opinion. First, the main purpose of using a water softener
is to soften hard water and solve the water quality problem of domestic water. Second, salt is not friendly to plants. If you need to water your yard, you'd better use water from other sources.