I read the link you posted and in it he mostly states that it is useless for livebearers and "the most commonly kept community fish of tetras, corys, angels and such, or even the Rasboras or most Anabantids". So he really doesn't state it is useless for all fish. Plus he goes onto say:
"It may even be part of the proper regimen for long-term maintenance of certain types of fish."
So he states it may be beneficial to use during an acute infection which in my mind makes me think that it could also help to prevent infection in the first place ergo being beneficial to add. But I'm not a MD or biologist.
I also have seen no signs that it is harmful to my fish so far. Alot of them even "suck" on the salt when it is added.