Thomas Dalton wrote:
This doesn't help ID the person (MAC tells you manufacturer, and sometimes model, but that's all). But it does help for blocking...
It helps to confirm that two people are the same. While a checkuser that shows that two accounts are being used by people with the same ISP would at best be a "likely", a checkuser that shows two accounts are being accessed via the same network card would be a definite match (assuming the rest of the address rules out a public computer).
On today's OS it's easy to change the PC's MAC. No doubt vandals will learn it fast. Even worse, if we treat as one-MAC one-user, and block by it, a vandal can vandalise with the mac of a legitimate user (one unlogged edition is enough to disclose it), which will found themselves blocked and appear as they were the vandal.