Good point. Another thing to consider if you're planning on a tent being waterproof in the rain is if it is properly seam sealed. Many are not. I seam sealed my own with a diluted silicone solution and paint brush, but you'll need a good day or more for it to fully cure. No way to do that on short notice.

I haven't had a problem with down quilt in the rain so long as its a good tent and properly seam sealed (I have a Henry Shires Notch) and you store the bag/quilt in a dry bag outside of the tent.