The vast majority of Webapps use the email-address for identifying their users.
As already mentioned, this makes the Twitter-API pretty useless for authentication.
Yes, we could ask for the email-address within our applications but what’s the point of doing this?
The efford to the user would just be the same as if using the regular login/registration form.
For my Website I now go with Google and Facebook only.
Maybe one day, you Twitter-Devs will see sense…