You can make a USB key with a unique serial number for very little money. Much better option than using the MAC address.
Much better for whom exactly?
It is safe to say that a very large percentage of all future sales will be for portable computers. In the case of an Apple Powerbook, having to carry around a USB key is not only impractical, but it also means you lose 50% of the available USB ports just so you can run the operating system.
