For me, an RSVP of "yes" means I will be there barring unforeseen circumstances.
Many years ago, for certain types of events, I used to RSVP "yes" and not show up if I changed my mind. I stopped doing that once I started hosting my own events and being annoyed when I prepared a venue, food, and drinks for a certain amount of people and a lot of items, food, and drinks were leftover because of no-shows. That taught me my lesson.
