Additionally, one water crossing taught me that gore tex is not water proof, it is water resistant. Your sleeping bags will absorb water through the gore tex and valise if laid in a puddle, swamp or mud (like all good patrol bases). This gives you wet sleeping bags, which are cold, and as an added bonus - extremely heavy.
As George said, this will keep the remainder of the contents of the bag dry when the bivy bag is out, and can be used for other things, like keeping your ration garbage in, and waterproofing radios/GPS etc. In the event of a fording or surprise helo casting, your ruck will float with a double bag valise - even if it weighs 80 - 100lbs dry.
Most people I know double bag the sleeping bags in garbage bags or some of the commercial substitutes (seal bags etc.) If you fall through the ice, or get hypothermia, the sleeping bags will save your life, so it is best to care for them.