One other thing that often is looked over with a QT are the corals, and other inverts.
Corals and inverts should go through QT. Red Bugs, AEFW, zoanthid eating nudibranch are just some of the possible coral pests that can ruin a tank. Not to mention ICH, and other parasites can come in on almost anything.
If you QT your corals and other inverts you can insure that none of these make it in to your DT. One other side benefit of a coral QT is that you can acclimate your new corals to the type of lighting you are using in your DT. When you move them to your DT, you do not have to go through so much dimming of your DT lights.
Clearly you can not use a tank that has ever seen copper treatment, and additional lighting of the quality of your DT is costly as well, but the safety net provided by putting the other 1/2 of the tank inhabitants through QT will save you headaches and live stock losses in the long run.
IMHO It is our responsibility as pet owners, to protect our pets to the best of our ability. In that way proper QT practices are the same as having a dog or cat vaccinated, and checked out before bringing it home to live with you and your other pets.