I did a little research on the subject once. The articles for it vs against it ran about two to one.
I talked to a guy on the phone from Aquarium Pharmaceuticals and he gave me some very convincing reasons why I should use salt.
Now granted, he worked for the company that sold the stuff, but he did make some good points, imo.
And for what it's worth, you can use Kosher Salt from the market, much cheaper than the AP stuff. Not good for scaleless fish like Catfish. I have a 7 incher in one of my tanks. If I add salt there, I usually go with half the recommended dose.
I've used it off and on, mostly off. I've never had any disease in my tanks either but I attribute that more to frequent and sometimes large (50%) water changes.