> It's logically equivalent to having a backup key, but it's more secure because if you lose a key, you can use another key to disable the lost key.
That's slightly more convenient but I don't see how it is more secure. With one key that has backups if I lose that key I can use one of the backups to disable that key.
Multiple keys is slightly more convenient in that scenario because with multiple keys I just have to disable the key that was lost, and then make a new key for the device that held that key and install it. With one key on multiple devices I'll have to install the new key on all of them.
Convenience is a key aspect of security, but consider the scenario where you have to replace all your locks while you issue a new key... you have to keep the extant key valid for a longer period of time.
That's slightly more convenient but I don't see how it is more secure. With one key that has backups if I lose that key I can use one of the backups to disable that key.
Multiple keys is slightly more convenient in that scenario because with multiple keys I just have to disable the key that was lost, and then make a new key for the device that held that key and install it. With one key on multiple devices I'll have to install the new key on all of them.