The nominal value of bitcoin will increase/decrease according to the laws of supply and demand, thus as banks adopt, the nominal value increases. The USD, when first adopted, had a nominal value likely in the low single digit million (
http://www.banknoteden.com/USA_Colonials.html). Now it is easily measured in the trillions. As of April 2013, the monetary base was $3 trillion and M2, the broadest measure of money supply, was $10.5 trillion.
In addition, it's the blockchain technology that we're proposing would save the banks from a risk perspective, not the actual BTC themselves.