I can mention at least one case where the supposed inability of key duplication might be a big problem. Several hotels that I know of have safes in the room, which lock with a Medeco key. Hotel guests are urged to stash their wallets, cameras, whatever in the safe, and take the key. However, if every person who's ever stayed in that room might have a duplicate key, (or if the housekeeping staff could have duplicated several of the keys), then there's a bit of a hole in the security. Granted, the thief would still have to get into the room. But the reason the safe is there in the first place is to protect valuables that the room itself isn't enough security for.