I think the reason why RSVPs used to be so important is that before the Internet became widespread, sending out invitations to many people was a bit of a hassle. People had to respond (even if they weren't interested) because to do otherwise would lead the inviter to think that you possibly hadn't received their letter - also it shows appreciation for the effort put into sending out the letters in the first place. Explanations aside, I do think it's a shame that people don't always respond to invitations. Even if you're not interested in going, it is a way of saying thank you for being invited in the first place. EDIT: I like RVOM. At the very least it'll cause people to reply asking me what it means.
We recently threw a party that required an RSVP and we were astonished by how many people did not respond either way. Many of them showed up, but never bothered to tell us they planned on it. There are a lot of things that go into planning a party and it's helpful to know how many people will show up. I like he RSVP.
because to do otherwise would lead the inviter to think that you possibly hadn't received their letter
I'm increasingly thinking that people don't get my email (much the way I seem to miss interesting hubski posts). What I send might get put in a spam folder or buried in a bunch of other mail. If I haven't seen a person in a few months, it's possible I do not have the right email address. All this obviously points to using the phone and leavig a voice mail. I'm not sure that's necessarily any better.