In the professional realm, public aquariums and associations, a qt (hospital tank) is a proactive solution in eliminating expected diseases and parasites. A holding tank usually comes after proactive qt where the animal is held for further observations with limited to no prophylactic treatment. Expecting an animal to be fully cured of the myriad of common diseases by maintaining proper water conditions is a fallacy. Once a fish has been compromised it is extremely difficult and stressful for them to naturally recover without the aid of medications. As for fw
dips, which is a Standard Operating Procedure in just about every public aquarium and association, if you adjust the pH and temperatures accordingly then there shouldn't be much more stress than what the animal is already going through. I have dipped countless seahorses, butterflyfish, angelfish, triggers, puffers, fairy wrasses, etc. and have yet to lose a single one to the procedure; It's taken roughly 5-10secs. for an animals breathing rate to return to "normal" during the treatments I've given. I understand the concern, but there really are more benefits to freshwater dipping than to not. Here's a decent article: Dips/Baths
and by Steven Pro (scroll to dip): Marine Ich/Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part II by Steven Pro - Reefkeeping.com
and seemingly most of your fish disease/vet health books incorporate them for recommendation.