Many keys (to rooms and buildings) are still tangible objects, where the tangible properties and affordances imply certain ways of usage .Who has not gotten a hotel key that you hand in at reception, because it is too big to be carried in a pocket? Moving digital many keys we get lack craft and unique affordances as they are just plastic cards or RFID tags in a specific form. With moving towards biometric authentication it seems that the key is intangible (so we loose options in the design space) but embedded into us (which opens up new possibilities).
The major drawback of physical and tangible keys is that if you don’t have it with you – when you are in front of the door they can not help you. Even if you know where the key is and you communicate with the person having the key.
… but thinking back a few days to the visions in Hiroshi Ishii’s keynote its seems that this is very short term problem. Having atoms that can be controlled (tangible bits) we can just get the data for the key from remote and reproduce it locally. With current technology this seems already very feasible – on principle – ( some Person uses a 3D scanner, e.g. embedded in a mobile device that has a camera and communication) and the other person has a 3D printer/laser cutter. Still the question remains if moving to digital keys is not much easier.
However if you do not have the key – and even so there is a solution “on principle” – it does not really help ;-)