I happen to think that straight into the DT is about the worst thing you can do for any fish. It's not just disease, but acclimation and acclimatization (they're different). Asking a new fish, stressed and probably malnourished, to survive in a display with resident non-stressed and presumably well nourished fish is asking a lot. The possibility of disease aside, I know any newly arrived fish going straight into my display would be beaten to death. QT gives a fish the chance to recover from shipping, eat uncontested, and get acclimatized to water, foods, etc.
Specific to leopards, I also put them into QT; however, in my case it's a 30 gallon tank setup as a fully functional reef tank with rock and sand. I've got an ornate in there now that is very shy still and clock challenged. I've been able to coax it into eating brine, mysis and sushi roe after two weeks, and it's out for a good chunk of the day, but it's been a battle. I'm absolutely certain had I plopped it right into the display I'd have never seen it again.