It's probably a minor issue, but if you can check for existing user names, you have a much easier time trying to find the matching password. Also, it could be a privacy issue (knowing user x is a customer of service y). Showing a photo as well...
Of course it depends on the context. On Hacker News all user names are visible anyway.
(edit) In a context of a system that allows anyone to register new account, which appears to be what the linked article focuses on.