I think there's a problem, though, with nomenclature. I consider a QT to be strictly for quarantining and when you medicate, you're doing a hospital tank. I keep the two separated and rarely QT unless I have some reason to. The reason I don't QT is that I think that most if not all fish have a bunch of diseases already in them. QT is stressful to a fish and results in some of the diseases being able to get out and attack the fish due to depressed immune systems. Rather than QT, for most fish I'll do an isolation tank instead, where it's connected to my system and has a lot of rock for it to hide in and graze on and no lighting. This allows the fish to relax and adjust while its immune system recovers. When I do QT, I always have sponge filters sitting in my sump to use on the QT so that I have some bacteria in there. Running a QT without any sort of biological filter is a toxic mess waiting to happen, at least in my opinion.
When doing a QT, really, it's an observation system so that you can see what problems you need to treat. Treating prophylacticly (sp?) is not the best because you're not sure what you're treating for. Again, though, to be fully effective, you'd need to QT EVERYTHING, rock, sand, plants, inverts, etc.. Even then, though, who's to say that when you introduce the organism, whatever it may be, it won't still be carrying some sort of pathogen?
Hard question to answer here, but really it turns into a matter of opinion. One person said there's some that never learn - true - but some may never need to learn because of experiences that indicate you shouldn't qt.
Regardless, it's always a good idea to be prepared in case you need to do something. One philosophy doesn't always fit one person's tank every single time.
