Another Nexon Mistake

That's all very well when you're designing a security system, but for the end user, obscurity is the only security you get - if someone finds out your login information, they can perfectly imitate you, whatever form that information takes. So ideally you want to keep as much of it secret as possible, frequently change whatever possible (so it's harder for someone to collect the full set at one time), etc.

Your logic amounts to someone who looked your address up in the phone book being able to burgle you at will.
They still need to either break in, steal your key, or get you to leave the house wide open.

This is no different from the hundreds of ways someone can find out your home address.