I think the people in this thread saying things like "getting an email account is easy" need to spend more time observing some non-tech folks using a computer. Because actually that is a substantial barrier for ordinary people.
My service accepts both an email address and a membership number, and this helps enormously with account recovery in the astonishingly frequent case that someone changes job or ISP and loses access to their original mailbox. And it's amazing the number of people who mistype their email address.
For sign-in, we accept either and simply detect which one was used.
This also helps us to support multiple profiles per email address, which is a common case for families using our service.