The majority of our staff don't have Outlook, just voice messaging, so a paper invitation was the most reliable way to reach everyone.
Actually, the invitation didn't say RSVP, there was a tear-off section on the bottom with 2 sentences: I WILL attend and I WILL NOT attend with a little box to check in front of the appropriate response. And at the top of the invitation it had all the details (date, time, location, etc.), it said "please reply to twhfan by X date" and it even gave them the option of voice messaging me if their little fingers were too tired to complete the response form.
So there was no chance that staff didn't know what they needed to do. I know, it's a petty little thing but it just bugs me. I play this goofy little scenario in my head where I'm sitting at the reception table looking and sounding like Prudence Goodbody, handing out name badges as people sign in, and when someone who didn't RSVP arrives, I wag my finger in their face and say, "Sorry dearie, but I wasn't expecting you. Back to the office and get to work!" It makes me chuckle as I imagine myself looking like Prudence

and keeps my blood pressure from going too high.