I have a, still empty, DT but two clowns in a fish QT and 6 frags in a fishless QT.
I was on the fence about the QT thing, but I'm glad I did it, even if I now have FIVE tanks set up around my house (DT, fish QT, fishless QT and two for TTM). The two clowns seemed just fine but after a preventative round of copper and other meds one of them still got ich or velvet (hence the TTM set up). So I'm glad that wasn't in the DT. Also, I put a picture up here of one of my frags to ask a question and someone noticed a vermietid snail, that I always thought was just a polyp. So one less hitchhiker to make it into the tank.
Granted, getting ick or invasive snails in my DT from almost day 1 wouldn't have been a big enough problem to cause me to get out of the hobby. I have enough money invested here that, at most, I would have just drained the tanks, cleaned everything and started over. But, if I were, say, several years into it and maybe my interest was waning, I could see ich or snails being enough to make me decide to be done with it.