Well, I have been on all sides of this issue in the past. I have been in the camp where I felt quarantine was too hard on fish and in the camp of quarantining fish without treatment unless the fish shows signs of disease. After dealing with crypt in my display, I have come full circle and firmly believe that you have to profolatically treat each and every fish for both crypt and flukes before adding the fish to the display whether or not a parasite is observed on the fish. Quarantine can fail b/c quarantine is nothing more than issolating the fish to see whether a parasite emerges. Sometimes, a parasite does not show itself even after a fairly long quarantine period. Other times, one fails to notice the parasite in quarantine and inadvertantly infects the display. Therefore, profolatically treating every fish in a hospital tank before adding the fish to the display is really the only reliable way to avoid introducing a parasite into the display when adding a fish.
It sounds like a real pain, but after living through the alternative of having my display infected with crypt, it is not so onerous. Moreover, quarantine lasts 6-8 weeks. If you profolatically treat, you can have the fish in your display in 3 weeks. Shorter period in a quarantine/hospital tank is very beneficial b/c the most difficult issue is usually maintaining water quality in quarantine/hospital tank. Therefore, the shorter the stay the less likely of having water quality problems in the qurantine/hospital tank, and you get to enjoy your fish in the display much sooner. I treat every fish, including sensitive fish, with cupramine and prazipro. I have treated mutliple puffers, wrasses, and tangs this way without a problem. The key to treating sensitive fish to to very gradually ramp up to treatment strength and maintain levels on the lower end of effective treatment strength. To me, this is the best approach.
In fact, I am in the process of doing so right now. My new trigger has never shown any signs of a parasite or disease, but he is going through cupramine for 3 weeks and at least 1 week of prazipro.
http://www.reefcentral.com/forums/showthread.php?t=1854664