Maybe if you have $20k and a decade of time in your tank, and you invest in maintaining a mature QT (or two, one with copper and one real, healthy tank), then it makes sense. But if you can't go whole-hog on QT, I'm convinced it's not good for your new stock and I'm not convinced it's even protective for existing stock in all cases.
This is wildly exaggerated. You can quarantine cheaply. A $20 20G tank heater and a sponge filter gets you going.