[tenebrisalietum]

Your "burned in" MAC address doesn't have to be the MAC it tells other systems in ARP replies, which is pretty much the only way a remote system can find out your MAC (essentially, by asking). It also doesn't have to be the MAC it checks when receiving incoming Ethernet frames.

Most uses of MAC spoofing are for privacy; so that entities that collect every MAC they see over a wireless connection aren't able to track your location.