When you bring a large number of fish into your DT without QT, it's just a matter of time before you get a sick fish. Parasites often don't manifest themselves until the illness is severe enough so there's no guarantee that every fish you buy does not carry parasites, even if you have a sharp eye and know how to pick healthy specimens.
Many people who first start out in the hobby do not quarantine and almost all of them get burned by ich, flukes, etc., and end up having to remove all of the fish into a QT to treat. Since they didn't have a QT to begin with, their QT aren't cycled and they have water quality issues, have to do large water changes (or not if they are lazy) and make mistakes along the way. The fish die as a result of multiple stresses, the aquarist gets frustrated and quits the hobby.
The above describes a large number of the hobbyists (hence the average lifetime of this hobby is only 1.5 years). The lack of strict quarantine protocols is the main underlying reason for this. Someone may get lucky and buy a small number of fish without having them come in with any parasites, but like i said, when the number starts increasing, it's just a matter of time before he gets an infested fish. murphy's law and mother nature always dominate at the end of the day.
Good husbandry (water changes, RODI, testing) do nothing to eliminate parasites. Once they are in, they are in. They can only be eliminated with medication, most of which cannot be used in a DT, especially a reef tank. a good quarantine process saves you A LOT of headache in the long run, including the trouble of tearing down the DT to catch fish. Take some time to read the stickies in this forum and the necessity of QT should become obvious.