We have a saying in our Scout troop... "if you're getting your sleep gear wet, then you're doing it wrong."
It's true. I have a trash compactor bag that acts as a waterproof liner inside my backpack. Then my down quilts go in Ultra-Sil dry bags. If it's raining while you set up, hang your tarp first, then assemble everything else under the tarp where it's dry.
So then you go to the question, "why down when it's more expensive?" Answer: weight and bulk savings. Down weighs considerably less than synthetic fill and it compresses smaller in your pack, which means you can use a smaller and lighter weight pack.