I think most of the time it will become a boring job,
Usernames has to hope on an email for the person to recall later. If not, it's rare for a user to use multiple usernames for a particular web application etc. Also, sending details back to the user's email gives a certain level of security it's very important point of both parties.
if we registering and create a username password for the website or application,it's too much clicking and filing, users not like to do it, so we can use easier way to give them a good user experience in our website or application,think about that,
i think if used to signup or log in using Google/FB/LinkedIn account etc,
it's really easier and user friendly for the user side, effective
An email address is easier to remember.it Should always be unique per user,
One less thing you need to ask them to register
and other solution is used an Open-ID,
"Open-ID is a safe, faster and easier way to log in to web sites.
it's an open standard that allows users to be authenticated by certain co-operating sites using a third party service, allowing users to consolidate their digital identities."(Wikipedia)