Well, like others have said, if you already have fish in your display tank, and those fish have never been quarantined, quarantining any new fish you get will be pointless. They will be disease free when you finally add them to your display tank, but since your other fish are not disease free, they'll just pick up whatever the other fish have anyways.
I've never quarantined fish either. And I've had fish get ich on occasion, but they've always been able to beat it themselves without showing any symptoms except the spots. As long as you can keep them eating, they will probably be okay. I look at it as something that is found in nearly every tank (the exception, as I said, are tanks of people that quarantine every fish, every time), and more as something that comes and goes but 99% of the time, does not present a problem to an otherwise healthy animal.