You wouldn't end up with a dead battery unless the car ran out of gas because; if it is not powered down, it is ready to run, just jump in and go. You only need the key if you power the car off. Other than that, as long as you keep gassing up, and don't power down, you would never need the key again.
At one time Toyota did supply a valet key with Smart Key systems. I had a 2004 Lexus that came with 2 Smart keys, a valet key, and a thick plastic credit card emergency card key. Without a trunk or a lockable glove box on the Prius it is easier to get by without a valet key.
