One of the biggest problems of conventional keys is the inability to limit the number of individual copies that are made. An employee or a tenant wants a key for his/her 'other ring' so they have one cut. Now the business owner or building manager has no idea that these exist. When you have this happening in an apartment complex or a business with many employees, the number of keys that are 'off the radar' can exceed the number that were legitimately issued.
The use of "Do Not Duplicate" blanks will keep this under control on a small scale, but duplicates can still be made, as it's easy for someone to disregard the request on the key and copy it without knowing what it fits. It's also fairly easy to remove the stamp altogether.
High Security Locks and Keys
Various manufacturers have designed locks with keyways and locking mechanisms that have been patented, as have the blanks that will fit in them. By maintaining tight control over who has keys cut, it eliminates the 'extra' key problem, as they are generally made available to tenants or employees by going through a single individual.
In most cases these locks also have mechanisms in place to foil picking or bumping attempts.