Actually, you can make your ground cloth a bit longer and use a stick, stake or bayonet to lift the top outer corner and you have a simple tent of sorts. Allows you to breathe while keeping rain off of your head. I have a buddy in my RevWar group who showed me this when we were campaigning. I made a normal sized ground cloth for normal packing about and the larger is used only for campaigning.
As to packing, you should decide whether you are making a knapsack or a tumpline. Either way, the ground cloth should be used to keep your blanket dry.
CS