You can always use "Forgot Password" if you mistyped a password.
You can't if you mistyped the e-mail address (the username you wanted is gone).
Now, when they realize they didn't get the email, they may notice the difference and they can fix it.
If they're unable to type their email address, and they're unable to recognize a mistake in it, then I don't see what hope there is for them.
Yes there are people who don't know about copy/paste.
Have a read of the comments for a sample of just how much people hate this.
Have you measured whether this feature actually helps anything? As the author said, people can see typos. But people who don't know their own email addresses will happily repeat errors.