it's pointless if you QT corals for white spots or velvet that potentially hitch a ride on it, if you don't qt your fish in the first place.
the chances of corals carrying fish disease is very slim. the chances of unquarantined fishes carrying fish disease is very high. so it's useless in QT-ing corals if no one here QT their fishes. unless you are QT-ing your fish as well, then QT-ing corals for disease is a good idea.
however, QT-ing your corals for coral disease and pest like red bugs, nudibranch, snails is a good idea and usually a few preventative dips in coral RX will do the trick. unless dealing with nudibranch eggs. then it's a whole other can of worms.