While the extra layer of anti-cheating is nice, I think the least attractive / Effective method is this Blurb of verifying Peoples XP Keys. Here is why (Some Already Mentioned):
A. Not many people are Cool with thier Os Key being Scanned and Sent to someone else's Server.
B. Said server, may gather a Large list of XP or other OS Keys (and that's silly, also prolly Warez land)
C. Many Warez copies of XP Float around, thus aqs mentioned, Tommy Mods getting banned, also takes out all of your friends in Georgia
Like Ae said, if there is going to be an Email Authen setup for Playing MTA, I could easiely see it blowing new Players off, Email servers being overused and exploited (Hi Signup Bot), and prolly a few other layers of discomfort.
Keep in mind while locking down modders is nice, you need to lookout for the End user connecting to your Server too, as they don't want to jump thru loops to get a Few Frags in.
The MAC Addy ban sounds best so far. As mentioned, yes, people can spoof thier MAC, but how many modders will Know how to do so / go thru the hassle of doing so, just to be a modding whore? For that rare case, I'd just ban the subnet for a night or 2.
Good Luck on the Anti-Cheat Magic, just keep the Players in mind, and try to respect thier registry as much as possible.