I disagree. If you had a tank with no fish, and no hosts, ich would not be present, but some fish can carry ich in their gills and if healthy not exhibit symptoms. Invertebrates are also known to carry it. Stress thins the fishes protective coating and gives the parasite access to the fishes flesh. A QT tank, if not properly setup will just stress the fish more and amplify problems. I have no QT tank and haven't had a outbreak of ich in 20 years. Oh I guess a copper treatment or something else to sterilize the fish first might work, but again, i am more affraid of the cure than the disease. in the wild, fish are constantly exposed to parasites.
Thats the problem with ich and surgeonfish (tangs). its a double whammy, they have a already thin slime coat protein barrier and are easily stressed.
Good read on this: http://www.skepticalaquarist.com/ichthyophthirius
Each to their own.