Mobile operators already use this feature. By reporting the IMEI number of your stolen handset it will be recorded in the Equipment Identity Register (EIP) to prevent the use of the stolen handset in the network. After ensuring the device is stolen, the IMEI is transfered to Central Equipment Identity Register (CEIR) which is a database of handset on a blacklist and it will render the device unusable on all other network providers as well.
Many carriers, including AT&T and T-Mobile, are now using this shared database inside the U.S and Canada. As Rory mentioned, most European carriers are Europe-wide so this feature will work across Europe
I would also like to add that the killswitch is different than IMEI blocking. The main goal of the killswitch is to render the phone completely useless by remotely erasing all data from it and therefore turning a phone into "paperweight". As for the IMEI blocking, it only prevents the handset from connecting to the network (IF the specified network is using the shared database for blacklists)
The terms "paperweight" or "bricked phone" refer to a handset that has lost all operating system data, partition data, and bootloader data; preventing someone from re-flashing the OS on the phone. The handset is completely unusable, even for offline tasks such as taking pictures or running apps. In such a case the phone turns on, but constantly displays the manufacture logo screen. It literally has the value of a "paperweight"
Apart from rendering your phone completely useless the killswitch makes sure your sensitive data does not fall into the wrong hands.