I covered it in comments to the last post: the problem is it's easy enough for spammers to get codes (between people posting lists of invites and the "create an account, age it and wait for it to start generating codes" tactic) that it's not an effective deterrant and it takes about as much administrative time to deal with invite-related problems as it does to deal with spam so it would just be a giant pain in the fucking ass for no net benefit.