well if they aren't real email accounts, you could always turn on registration emails. It will send an email to the email addy, and if they don't respond in 24 hours their registration is denied (or discontinued) at the server. This would reduce the number of registrations from the same IP. ( At least in my opinion) one reason to limit this (IP's or Emails) is to reduce the number of spammers that can get on....however, this is becoming less effective as big spammers are now using live people (and paying them) to go around captcha and other things designed to keep bot spammers from joining.
Dunny
