The fob without battery most definitely works in the slot. Otherwise, if you were to run out of fob battery, there would be no way to start the car. You do NOT need a battery in the fob to enter (use key) or start (fob in slot). This is why it makes a perfect situation for that rarely-needed fob that is stashed.
Finding this behavior interesting, I read it to my wife only to see she was bored by it. When I ask why, she told me how just yesterday she forgot her purse was in the car and finally locked the car using my fob!