I totally agree with you, and personally do not add salt on a regular basis to any of my freshwater tanks.When a fish is in a tank with clean water, no parasites, is not stressed, is not being moved, has no injuries, and is feeding, the value of salt is much less supported.
Having said that, certain pathogens/parasites are often found on a regular basis in most tanks, and only surface when a fish comes under some form of stress. F. columnare is a perfect example of that. From the link that I previously posted:
So how does one know exactly what is lurking in their tank water at any given time? Regardless of when they last introduced a fish, plant, a piece of driftwood, etc. For most hobbyists it would become a case of probably never being 100% certain whether there is an absence of need, or not.The bacterium can keep its infectivity in lake water in laboratory conditions for at least five months
For those that are buying new fish on any kind of regular basis, I'm thinking that a low dose salt regimen might not be such a bad idea. Case in point, see post #21 from the following past discussion.
http://www.monsterfishkeepers.com/forums/showthread.php?527245-COLLUMNARIS-DUCK-LIPS-DISEASE
As an example, if I was importing and/or breeding certain species from certain areas known to carry some of these pathogens, I would most definetly be treating my tanks with a low dose of salt (1%) and/or with copper. In this case the pros would outweigh any potential negatives. Some species that come to mind are some of the highly inbred designer lines of fish, such as discus, flowerhorns, EBJD, etc. I don't personally keep or breed those types of fish, so for me personally it's a non issue.