Why did the fish die in you QT? Was it because the tank wasn't setup probably or you let the levels go over to the point that the conditions were bad and the fish died from poor water quality. If you setup the tank correctly, and treat using copper, at the correct level and for the correct time period, there isn't any reason that you couldn't get rid of the Ich and have the fish live. No reason at all. As for using things that clean the fish, well most, if not all, of these things don't eat Ich or if they do, they cant' get into areas where the Ich is. Also, a lot of fish don't like being cleaned, especially since most of these cleaners don't use remove parasites but scales and body slime. This actually makes it worst for some fish.
As for not QT all fish, well that's very bad advise. Also, just because a fish is eating or behaving good at a LFS doesn't mean it's doesn't have some sort of disease, which just may not be showing at the moment or even after a few days, and lets not forget that in those few days more fish are added to the tank or their system, which can very easily lead a fish to be infected. So, basically you may have brought a fish that was fine a couple of days ago but now is infected, and you're going to put it right into your system.