On my form ( http://wolfsnap.com/wc ) I have a field that is sometimes hidden that asks the person to 'join my mailing list'. By default, it's checked. I use that field to determine whether or not to forward the info to mailchimp.
Now, in some cases (like when you 'open a trouble ticket') that mailing list item isn't displayed. But since it's actually "checked" by default, the mailchimp addon still sees it as a valid entry.
However, this isn't consistent with how other hidden fields work. If I have a field that's "required" but hidden, the form submits just fine. Also, if items are hidden, the paypal addon works as expected. But, the mailchimp one does not.
All plugins are up-to-date.