Mostly it's used by spammers. When you use it there's a hard to shake impression that you don't understand the point about getting confirmation by email from the email address owner, and that you might be using eg checkboxes on a webform as a confirmation.

That mailchimp blog? It's wrong. What they describe as confirmed opt-in is not confirmed opt in, and what they describe as double opt in is in fact just confirmed opt in.

If anyone from mailchimp is reading: please fix this fucking annoying and stupid error.

EDIT: That constantcontact post is correct. Notice how they put "also known as double opt-in" in brackets, and then never use it again but only use confirmed opt-in?