You're right. QT won't catch everything. But not using a QT won't catch anything. Almost every parasite problem on the disease section of our forum could have been prevented by a good QT routine. Very seldom does a parasite problem ever appear after all fish have gone through a QT. Even then, the parasites can often be traced back LR, even coral, etc., that weren't put in a QT. Often, fish simply aren't in a QT long enough.
Public Aquariums do a lot of things that are just impossible/ impractical for the home hobbyist. Chloroquine phosphate is an excellent anti-parasite drug, so is quinine sulfate. but they are very hard to keep is suspension and its impossible for us to measure the actual water concentration of the drug. . Big facilities, I assume, have labs that can do this.I know a couple of SW aquaculture labs have this ability. We can use copper, hypo, or tank-transfer in a QT because we can very accurately measure Cu or SG. Dealing with parasites is a major PITA; but they don't have to be. UV & ozone may help a little and are great water clarifiers. However, IMO & IME, neither will prevent of cure a parasite infested tank. Neither will diet, garlic, any "reef safe" med, etc. A HT/QT and a fallow tank are the only way to permanently eliminate parasites. Parasites, like ich, may be manageable for a while, but can become a disaster as soon as the right stressor/conditions, or just timing come along. I can remember every damn detail of tearing apart a big tank with ich and finally getting rid of it. I haven't sen any contagious parasites in any of my tanks almost 20 years since I did this and became started to QT everything.
BTW, you mentioned DD and I think they have the best fish on the planet. Even DD strongly recommends that you use a QT regimen with their fish. here's another thread on the same subject, read post # 11 by hvacman250, this type of experience is very common.
http://www.reefcentral.com/forums/showthread.php?p=19634484&posted=1#post19634484
