There are
freshwater fish that come from naturally brackish areas that probably enjoy the salt, but ultimately it comes down to: There are people that believe it is unhealthy for a lot of freshwater fish and there are people that believe it is healthy for most freshwater fish. People from neither side waiver too much in their belief and so the arguement will probably go on until scientists find a way to prove definitively which fish benefit and which do not and which are harmed by the use of salt in the freshwater aquarium.
I would be interested to see, but will never do it because I believe salt for most freshwater fish is not a good thing, a fish kept with salt and a fish kept without salt. The fish being exactly the same type in a tank by themselves, exactly the same size tank. Exactly the same care given to both tanks etc. And have the experiment done with say... 100 with salt 100 without salt (would take 200 tanks

) to prove one way or the other for that type of fish. I wish it could be done without the possibility of harming the specimens, that is why I will never do it.
(Plecos haven't the slime coat to do well with salt.)