Typically, MAC is assigned by the hardware manufacturer (sometimes you can change/spoof it). IP is typically assigned either by the network administratior or DHCP. If all addresses were based on mac, you could easily intercept the messages from anyone. IP allows a layer of abstraction in which you can group multiple subnets into the same external IP or set up subnets within those subnets even.
Another problem is that numerous devices will share the same MAC address, it's just statistically unlikely to find them on the same network. It's an especially big problem with cheap/knockoff network equipment. See: http://www.linuxquestions.org/questions/linux-networking-3/m...
Another problem is that numerous devices will share the same MAC address, it's just statistically unlikely to find them on the same network. It's an especially big problem with cheap/knockoff network equipment. See: http://www.linuxquestions.org/questions/linux-networking-3/m...