It doesn't make sense if you don't have many snakes, however if you are a big breeder and have enough snakes to even have a QT rack, even if you lost that rack you still have your main collection staying healthy. That at least takes away risking your whole collection, Yes you could lose your QT rack, but if they have a whole rack for QT who knows how many racks they have for their "main" collection.
But why danger your snakes in the qt rack also? The snakes in the qt rack can have some high end morphs. Why care less about another snake rack that you might have invested a lot of money in?