When you try to sign in using an incorrect email address, the website gives you an immediate feedback about that (top), while it displays your avatar picture if you typed in a valid address (bottom).
|An example of Quora's immediate and clear feedback.|
Getting feedback early saves you the time you would waste by typing a password while trying to access a non-existing account.
Of course, you could play with that feature to see which email addresses are used by other people to access Quora. And if you were an evil spammer, you could use it to harvest email addresses. But you could do that with Gravatar as well, right? ;)
If you think about this kind of feature on a username-based website, this may add even more value, considering that you may forget your username even more easily.