This isn't something that happened to me but to some parents that had a bday party for their son that my child attended. The parents sent out invitations over a week in advance and made it VERY VERY clear that for planning purposes, to RSVP and RSVP BY A SPECIFIC DATE.
Apparently, there were about half of the people they sent invitations to that did not respond one way or the other. So the parents extended invites to more kids whose parents confirmed they would be there. On the day of the party, the kids whose parents didn't respond showed up, AND the kids that got the invite in their place also showed up.
The following is why people want you to RSVP for things like this:
*Not enough party favors/bags for all the kids
*Not enough cupcakes and ice cream cups for all the kids
*The host parents had to come out of pocket more money to the establishment where they had the party for the additional kids whose parents didn't respond to the RSVP request.
I told the dad "You're a better man than I am. I would have told them they are more than welcome to stay and attend, but they need to pay for their child and due to them not RSVP'n (like we asked), there's not enough party food and party favors to go around." He said "If my wife weren't here..." LOL!!!