Simply put, you either set up a proper QT to deal with the size specimens your acquiring (doesn't need to be as big as a long term display tank) or you play Russian Roulette.
Personally, I QT. In the long run it actually requires less effort and less patience than dealing with disease issues in the display tank.
I agree. I also keep a hospital tank for treatment if a new fish "flunks out' of quarantine.