Depends on what you mean by QT.
I have no LFS near me, so I buy fish online. Sight unseen. No idea what I will be getting, aside from a WYSIWYG picture. These fish go straight into what I call QT, for 30 days. I don't treat prophylactically, I just observe. During that time, sometimes the fish shows obvious symptoms of something, a wound, a parasite, velvet, worms, etc. Then I will treat it. And after treatment, I will let it spend another 30 days in observation. If I could observe the fish before I buy it, I could weed out 90% of these cases - but I can't do that. So I observe it at home.
Most of the time, the fish spends those 30 days in the QT tank getting used to this environment. I have no idea where the fish has been before it gets here, how long ago it was in the ocean, what it likes to eat, nothing. It just makes sense to me that, before I throw it into a 220g tank with 30 other fish and let it compete, I give it a chance to settle down, start eating, maybe fatten up a bit.
I am not trying to create a disease-free DT. I know I have ich in my DT. Haven't lost a fish to it. I know I have kept velvet out of the DT because I caught it in QT - not sure how many fish I would have lost then?
I do agree with Paul on one very big point he hammers: I feed high quality food, often including live black worms, baby brine, copepods, live clams, and for certain butterflies, live coral (ouch). I think that has also made a huge difference.