There are people that QT everything, and although you cannot medicate the CUC for any diseases such as ich, you can wait out the life cycle of the ich so that you are possitive you are not introducing it into your system. The ich's life cycle is six weeks long, so if you could keep them in QT for six weeks without fish going in, that would be your only advantage. This is something you have to decide on if you are going to go for or not.
I do have to say that I do not practice this, but I have to disagree with the statement that there is no reason or benefit of QTing the CUC.
This is a great question though!