Legitimate Uses
There are definitely legit uses of spoofing, case in point:
I use Verizon DSL, when I first moved in I was having problems connecting to the Internet, so I called them up and asked them what was going on. Turns out that my connection was bound to a the MAC address of a NIC I didn't own.
I tried to ask if they could release the MAC address, but they made me call back and try to see if the DHCP lease would simply timeout, before finally giving me the MAC address, so I could spoof it on my router.
Quality service right there. This is the same firm that responded with, "I don't know, we do it by county in Ohio," when I asked a question about my phone service in California.