Doesn't that just give loopholes for the cheaters to fall into? IE: Servers with less capable managers? A global MAC banning system ensures that anyone running MTA is safe from that 90% of people who just google "MTA HACK" and download the latest "S0beit sploiter"
... It would seem I've coined a new phrase!
Anyway.. I don't see the detriment in making it global, since the only possible thing that could happen, is the modders would just wreak havock on the next server, and the next, and the next, and the next. If it's global, then it's just one discovery needed to rid them for good o.0!