Petty tyrants are invariably the worst. It's not your network. It's the company's. Your job is making sure they can use the network to do theirs. If the user is responsible and the personal device will improve their productivity, everyone benefits if you green-light their access. You might even learn something. Possession shouldn't extend beyond personal pride in your work.
of course the would be attacker would have to know WHAT MAC addresses to spoof in order to get access. Not that it's that hard to figure out but still. Also you would need to be on when that other machine isn't or problems will show up. Of course this is first line of defense, not the last.