I don't get them. And I had frankly never heard of them before I came on TK.
What I really don't get is, if someone isn't important enough to you to include on the first round of invites, why are they suddenly important enough to invite when others RSVP no? What exactly causes this level of importance to change? You obviously didn't care enough about their attendance in the first place, so why do you care now?
Is this need for a B-list fueled by the bride's desire to look like she has a lot of people who care about her enough to come to her wedding? Is it a status thing to want a certain # of people at the wedding? If you're on a tight enough budget to require not inviting everyone in the first place, wouldn't it be more cost effective to keep the no RSVP's instead of replacing them with more people?
This is obviously not directed at anyone in particular, but is more of a rhetorical question.