Boy this is a highly debated topic. But I see a few statements that aren't really true.
1. Sodium and Chloride are not found in nature. Actually they are. And in significant concentrations. Here is data on mineral content of rivers throughout the world.
http://www.fao.org/docrep/003/T0537E/T0537E02.htm
Sodium has a concentration more than double the calcium and iron concentration of "soft water" rivers like the Rio Negro. It's considerably higher in rivers in Malaysia (where many aquatic plants we keep come from. Chloride is found in even higher concentrations.
2. Chloride is not required by fish. Not true. Chloride is a vital necessity in fish respiration. The chloride cell is the basis for gas exchange and osmoregulation in fish gills.
https://books.google.com/books?id=H...epage&q=fish respiration and chloride&f=false
As in all things, an excess of anything can be detrimental. In the case of aquarium salt (which is just straight up Sodium chloride), small amounts that produce levels close to natural water chemistry would be fine. But since hobbyists have no way to measure this accurately, and since most tap water is probably there already, in nearly all cases the salts are being overdosed compared to natural levels.
In my personal opinion, unless you are treating fish for a specific parasitic disease that is vulnerable to increases in sodium chloride content, there is really no reason to add aquarium salt to your aquarium. Unless you are using RO water, there is already plenty of sodium and chloride in your water already. If you are using RO water, small amounts of sodium chloride (in conjunction with calcium and magnesium salts) should also be added to the water to remineralize it.
Andy