You do all realize this was introduced in a beta, right? As in, this feature may still change before the final release. As in, use the Feedback tool to submit feedback on this new feature. There are a number of cases where iCloud Keychain doesn't save a username where it should, making this feature really useful. On the flip side, as some have mentioned, a number of cases where a username is completely inappropriate. What I'd like to see is this same prompt, but with the ability to still save the login without the username. Based on the Twitter pic, it looks like you can't actually save the password without having a username attached to it.