Not sure you guys disagree! The parent said, "Maybe you don't display it as an "account", but I can't think of a single commercial website where you shouldn't at least be taking an email address as part of the transaction."

Then #3 on the bulleted list of six points says, "Username = Email. Don't make people remember a username for your site. You may allow them to pick a username later on that can be used in lieu of their email address, e.g. as the URL for a profile page, but don't force them to use a username to log in."