I didn't think I needed a quarantine tank until the Great ICH outbreak of 2007. I bought a clownfish from a dealer who I later found out is notorious for having sick fish. All my fish got infected and I ended up losing a couple. My main tank now has Ich and I can't medicate it because of all my inverts. I would run fishless to get rid of the Ich if I could catch the fish who survived. So now my main tank has ICH and it probably always will just because I didn't get a QT tank right away. So, if I had been a little patient about setting up a QT tank, I wouldn't have ICH.
I also have the benefit of knowing that if I fish does get sick, I have a separate tank to put him in.