There are many freshwater fish which are not tolerant of salt...types of Tetras, scaleless Catfish, etc...will not deal with it well. Low dosages might not be the end of the world for them...but it is not recommended and really serves no purpose unless you are treating a specific illness.
Saying that you use salt and have never had Ich, would be like me saying I've never had a fish with dropsy...so it must be my ceramic octopus decoration preventing it. There's really no relation. If you look at the life cycle of Ich...it is only susceptible to salt at certain points...not as a full time preventative measure.
Salt can serve a purpose when treating specific illnesses, and also preventing nitrIte uptake when a tank is cycling. Again though, the type of fish in the tank must be taken into account for dosages.
This is another one of those fish-keeping myths which doesn't hold water IMO. In my local PetCo they have a little cup of aquarium salt in each tank, and proudly advertise "We use and recommend API aquarium salt in all of our tanks for the health of your fish!". In that case it all comes down to $. When I was a teenager I worked in a shoe store, and corporate was all about add ons. Selling a $5 can of water repellent to each customer adds up on the bottom line very quickly...but in this case it really has no bearing on the health or welfare of your fish.
Another point...brackish water is not created using aquarium salt...it requires products like Instant Ocean for marine tanks...a different type of product entirely.
One of the mods (rookie I think?) had a great line about salt...it's great for treating specific illnesses and for French fries...nothing else.
