While the article you linked to points out correctly why they aren't good as passwords, they also aren't good as usernames (though they may be good as an alternative by which a username is looked up, with a fallback to using the real username), since they can be destroyed.
Palm prints were used at a nuclear facility I visited as an alternative to fingerprints in step 2. After you scan your id card you scan your palm print and the system verifies the palm matches the one that is registered with the card. They are not as unique as finger prints but it makes it much more difficult to use someone else's card (or username).
While the article you linked to points out correctly why they aren't good as passwords, they also aren't good as usernames (though they may be good as an alternative by which a username is looked up, with a fallback to using the real username), since they can be destroyed.