I would dare say that this is when you have too much of salt in the system, or for too long. Because salt has always been a good treatment for parasites, bacterial infections and so forth. And in one of my books, in the sickness section (which a veterinarian wrote) salt is recommended almost always when you can't find a reason for the fish to be sick!
It relieves the pressure on the gills, for example, in case of treatment of wounds or water quality problems with nitrite, although it slows down the filter bacteria.
But yes, it makes the fish stressed when you add it and they're not used to it, but at the same time, it will take care of parasites or bacteria. It works much as when a person uses salt dissolved in water for desinfecting various wounds (like a piercing).
Overuse of salt has also created a few strains of more resistant parasites (Sorry, I can't remember their names) but in Japan, they use no other medication than salt, and let nature do the rest.
So I think this is not about salt as a treatment method, but more as in the use of marine salt. So YES, it shouldn't be used unless you're treating for parasites or disease. Keeping a constant amount of salt in the tank isn't very advisable.
Yey, you make many interesting threads!