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QT for corals is a risk v reward thing. Some people who collect expensive SPS like to dip and QT coral frags to make absolutely sure that no pests like planaria or monti eating nudis make it into their displays. If the display is full of big colonies and they don't add stuff very often, then I agree the risk is even less...but still there. My corals are all cheap and common.
There is also the risk, although it's really low if you cut frags off plugs, that anything wet can potentially bring in fish disease.