I wouldn't mind it if the keys would stay with you after death, like pop-tabs, but allow the enemy to create a mold of the key for your car.
It could be checked by when the enemy clicks to create the mold, it checks to see which vehicle you locked/unlocked last and gives the player a key for it.
The main reason I am against keys is because it is annoying when you accidentally lose them, by putting it inside the wrong safe, or inside your friends car.