When I first started 8 years ago, I didn't QT a single fish that went into my 32gal biocube. Never lost one either until a major alk swing that was likely the cause of a 6-line disappearing. When I upgraded to a 60 cube I was beginning to read about QT, but didn't until the last few fish.
Fast forward 3 years to my 220DT. I put the only 3 fish I didn't sell or give away when we moved from Atlanta to Orlando through TTM and have QT'd every fish since. I even have a reef club member down here who will QT fish cheaply, and still QT'd them on my own. The reason why is the shape the fish, especially ordered online, have been in when they've arrived. I lost 6 dispar anthias to what I now believe was almost certainly urenoma. I lost an engineer goby, purchased at a LFS, because his tail was slowly rotting away and nothing stopped it. And most recently I lost 3 lyre tail anthias to urenoma but may have actually saved the other 3, they're now in copper having been treated with metro for 2 weeks.
I can't imagine unleashing whatever these fish had on the 14 fish I currently have in the DT. Especially my clowns who were my very first fish 8.5 years ago.
While I truly wish the ornamental fish supply chain would get its collective act together and implement serious measures to assure that fish losses are minimized, it isn't likely to happen any time soon. So I QT all new fish in either 10 or 20gal tanks with simple HOB filters, air pumps, and power heads treating prophylacticly for intestinal worms and flukes with API General Cure, Copper for velvet/ich(this is the final step, 14 days of daily tested therapeutic level and then transfer to my 60 cube), and now certain fish like anthias will get automatically treated for urenoma. After 3-4 weeks in QT they go into the 60 cube for another 2-8 weeks before going into the DT.
I believe it be fully necessary unless you're making use of those who are trying to create a business of selling pre-QT'd fish. And even then, I'd observe them for at least 3-4 weeks before putting them in the DT. The only exception is small nano tanks.