I do not see the benefit of this from a usability standpoint.

From a security standpoint it seems like an absolute nightmare. If you lose the password to your email you could lose access to all your accounts.

Most people do not own their own email server and so this could have catastrophic security consequences.

Of course the sites could check for IP location consistency but in order to spot security threats early you would need to track logins to multiple sites. The only way to do this would be to either give another third party company the ability to track logins and implement fail safes. At that point it begins to operate a lot like the companies which allow third party authentication through Facebook/Twitter/Email.