We recently picked up an iPod Nano for my mother-in-law and had to fill it up with her music for her. Note: nothing illegal here; she gave us her CDs and had us transfer them to the iPod all legit-like.
After connecting the cable and seeing the little iPod Nano icon appear on the desktop (a nice touch in itself) we decided to register it. What I found particularly helpful is that the application automatically detected the serial number of the device and entered it for us. No searching around to find the number (and in the case of the Nano, no looking for a microscope to read it).
Since Apple stands to gain the most out of the registration process (being able to market to you in particular), they're smart to make the process as simple as possible.