It does nothing to reduce ammonia toxidity Coraline, it nitrite and nitrate that it reduces the toxidity of. You're right in thinking that it is a relic though, and LFS's exploit this stuff for extra cash, it's a nice little earner for them

Salt increases the pressure on the Osmoregularity system of the fish, and the long term effects of that are (as far as I'm aware) currently unknown. What is known, is that fish kept in salt for all their lies don't last as long as those that don't. To me that is reason enough to not use it

It is sighted that it can lead to premature organ failure, though again, I haven't seen any evidence to support this...
As a short-term med for whitespot with none-salt-intollerant species, and while cycling, it is useful, but other than that, it is doing more harm to the fish than good, though the extent to which as yet is not fully understood...
It used to be used, as in the old days, they diden't waterchange. This lead to nitrate build-up that would wipe tanks out without warning. Salt was seen to prevent this, though nobody realy bothered to look into why the tanks went over in the first-place, or the effects that salt would have on the fish long-term
HTH
Rabbut