I thought I covered that, what follows the line you quoted is what I'm refuting.
But there's another reason not to have it persist depending on the style guide for forms on your site (you are using a style guide, correct?). The style guide may say that incorrect fields be called out as incorrect, using an icon or a red border or whatever; the correct fields are not called out and do persist their values, so you only need to change the incorrect ones. With a login form, the entire form is invalid, so do you persist the values in all the fields but indicate that they're all bad? This may require a change in your site's style guide, to acknowledge the existent of entire forms that could have invalid data and are not in an editable state, but it's does require not just blindly persisting the login name.
