Being sure of someone's Digital Identity is only as important as the information you're trying to protect. Many online services can be adequately protected using simple usernames and passwords and most communications don't need to be encrypted. These are easily compromised, shared or copied but in most cases, the value of the information being protected isn't that important anyway.
Usernames and passwords offer One Factor Authentication because the end user is only required to have one item or factor: 'something they know' (the username and password). For other Factors of Authentication, you need to carefully balance the security level and Validation Process used, with how simple it is for the end user to understand and use.
The following sub sections may help to broaden your knowledge and help you to make the more informed decision: