The problem with that method of water softening is the salt itself. You are exchanging either a sodium or a potassium for the calcium or magnesium, that makes up your gH. Those salts that you put back in can have a negative effect on the system, Na much more so than K though. Water softening salts have no effect on kH, which is more important for plants.
If you're looking to house discus, you could acclimate non wild strains to your water. They can live a very healthy life in that type of environment, I've seen it first hand. Breeding them in that water would be a different story, you cannot substitue water softening salts for an R/O unit in that case.