I don't QT (but I'm a fowlr guy and not a reefer). While I appreciate the ideal of a QT and know that it is useful for certain tanks, I do believe that it can do more harm than good in certain circumstances.
Here are my problems with QT. First, they are almost always small, inexpensive setups. In most cases the fish will be introduced into an environment that is too small and has minimal filtration. A large part of the QT argument is that people want to go buy a 10 gallon tank and stick a hang-on filter on it. I believe that this type of environment will cause enough stress to harm some of the more delicate species such as powder & achilles tangs, idols, pinnatus bat, rock beauty angels, ribbon eels, etc. The amount of beneficial bacteria in the tank may or may not be sufficient to provide sufficient biological filtration to the QT'd specimens. To avoid the above problems, a QT should be just as large as a display, with enough filtration to handle heavy ghost feeding. However this is not how QT's are done in practice.
There are also pragmatic reasons for me not to QT. First, I don't have the room for another large tank, nor do I have the desire to maintain it the right way. Second, ich is not all that difficult to deal with. I have dealt with bouts of ich in my years and it's really not all that big of a deal. I think the stress of tearing apart a tank and putting a sick fish in a woefully small, cheap tank is far more detrimental.
This is my reasoning. If my tank were smaller, with smaller fish, or if I had a reef then I may have a different opinion.
