Maybe I'm misunderstanding. But to moi:

--A "save a date" card from the couple is a commitment that they will be sending an invitation.
--A "save a date" card does not require a commitment from the recipient to attend the wedding or RSVP. That comes from the invitation.

If someone sends me a "save a date" card and then doesn't follow up with the invitation, I'd consider them a little lacking in the etiquette departimento -- unless it turns out that it was lost in the mail. I'd say something very innocent when you see them along the lines of "Got my save the date card. When are you sending the invitations?"