Really, a MAC address is a unique name. Now your pissing land requires people give their name. I find my name has been banned, so I make up a new name. I think it is clear I know yo don't want me there, but I give a new name to get in.
They don't have to be unique. MIT/JSTOR wouldn't have really cared about accidentally blocking people unfortunate enough to share Aaron's MAC address. And, the MAC address would have been perfectly reliable had Aaron not spoofed it.
With enough resources it's practically impossible to provide reliably unforgeable credentials without strong crypto, and strong crypto goes against the whole point of MIT's open network.
Really, a MAC address is a unique name. Now your pissing land requires people give their name. I find my name has been banned, so I make up a new name. I think it is clear I know yo don't want me there, but I give a new name to get in.