Limiting accounts to one per IP address doesn't work. A large part of our player base still uses dialup, thus gets a different IP every time they connect to the internet, often they get one that was previously used by another player. Additionally, many ISPs (e.g. AOL) assign the same IP to several users at once, which would also cause a problem with this. Last but not least it is rather easy to use a different email address by connecting through proxy servers.
Since most (if not all) ISPs provide their customers with at least one email address (which can be used to sign up), it is the easiest and best way for us to enforce this rule. If people are too lazy to either create that email address (or one from their personal domain) or retrieve the password for it, then it honestly is their own tough luck.
Even if you don't have access to one of those addresses yourself, you can always borrow one from a friend or relative to sign up. Once logged in you will be able to enter a free contact address which will be used afterwards if there is need to contact you.
Final statement: This rule won't be changed in the near future, so either you take a little bit of effort into this, or you don't and probably wouldn't even last very long in this game.