From what I've read, ich grows in the fish (if it has it) then jumps off the gills into the sand breeds then the cycle continues, if is in the sand it can pass to the other fish then continue its life, in the qt the ich has no where to go so the goal is to have it die without breeding, once the fish out lasts the ich and gets better throw them in te dt.
Also in a qt you don't have corals so you can use more medicational techniques like copper, and other medications etc to help kill the worm, parasite, bacteria infection whatever it may be, also just to prevent it from passing through out your other fish,
Personally, it can be stressful on the fish. But I feel helping it is still better than taking the risk of it not being healthy.