Aykwa - the problem with that is that not everyone has a static IP address. Most cable internet services use a dynamic IP system which resets the IP of your house/residence internet every 4-6 hours. Your IP will remain the same on your local network, but your IP according to blizzard and according to the cable company will not always remain the same. This system keeps them from having slow downs... kinda like a soft reset to your internet (ever notice big lag spikes? that's them reseting your DNS).

If they had a way to verify other things, like the serial code of your windows OS, the serial code of your motherboard or even some other random piece of hardware, you could do this. but you would need to, first, have the permission of the user (easy - update the ToS) and second, get the rights to use that information for security purposes. Some companies don't want other companies using their serial numbers for their own purposes. This can and would be a bigger hassle then just giving everyone an Authenticator (it's free already if you have the worlds most popular smartphone or mp3 player).

P.S. The Authenticator is the same concept most large businesses use for their own employees for logging into their servers. If they trust it for secure files that actually mean something... I can trust it for a game.