When I started in the hobby I was doing "sterile" quarantines with tank transfer and prazi. Every fish I put into QT seemed stressed, I had trouble getting them to eat, their colors were always off. Several starved to death. No matter how careful I was I had ammonia issues. The one time I tried copper I had a bad outcome (this was before the Hanna Checker).
Since then I've done 4 weeks observational QT in a cycled tank with sand and LR. Since changing my methods I've had I think 2 losses in QT. One was a tang that I am pretty sure had velvet, although he was in a "sterile" tank because my usual, comfy QT was occupied. The other was a flame hawk, never really knew what happened with him. My method is to leave the fish alone, let them hide in the rock for as long as they need, and usually within a few days I have an active eating fish.
No outbreaks in either display tank.
My experience is only about 2 1/2 years, but that's what it has been. I don't have advanced knowledge of disease transmission, but it seems to me that if a single fish is in a small, closed system with a parasite, the parasite would replicate, overwhelm the fish and there would be signs over time. Especially as ca1ore stated, the process of capture - ship - acclimation is inherently stressful. And I DO know that there is no such thing as a drug without risks. For me, the risk vs. benefit of prophylactic treatment isn't there.