In the modern era, the RSVP has become a rare thing. The ettiquette that you need to respond has slipped away. It seems strange--as we’ve moved into an always-connected world where invites flow through the Facebook, the Evite, and other services--that people just don’t reply one way or another. It’s so easy now, you would think the reply rate would go up, eh?
My guess is that in the modern era, replying doesn’t usually matter. In the world of more loosely connected friends, larger pools of people on an invite, and fewer “formal events” where you need to make sure there is room at a table and enough food to be served, the need to have your guests RSVP has faded.
There are formal events (weddings, for example) where it’s extremely important to know how many people are coming. But the culture of not needing to send an RSVP has become pervasive. Even at weddings, people just don’t reply.
Who am I to judge? I’ve always made a point of trying to reply, but often my reply is “maybe."