Its from personal expirience of snails eating a colony of very expensive zoanthids, then an infestatioon of aiptasia, then flat worms...... after that I just started QTing everything, I have prevented aip from getting into my tank again at least 15X I have found sundial snails (zoa eaters), pyrimid snails (clam killers), prevented keyhole limpets....... there are about a thousand nasty reasons to QT and not one valid argument against it. (deciding to QT or not is each persons descision, i'm not insulting anyone that doesnt QT, I'm just saying there really isnt a tank health reason not to QT, and many reasons for it)
look at it this way, a chunk of LR that comes out of a tank that had ick 3 weeks ago can infest your tank with ich....... qting will prevent that.