I don't remember where I read about doing this, but it has saved me tons of frustration because on occasion I would have my foil sheets accidentally leak water and ruin my cheesecake crusts which were a real aggravation. No more of that! I've never been brave enough to use ONLY the plastic bag so I've always wrapped 2 sheets of heavy-duty foil on top of the plastic bag, but I don't see why 'plastic only' wouldn't work equally well.