The other day I was having a conversation with a friend about guests who don't show up to your wedding or event after they sent their RSVP indicating they will be attending your event. I've attended countless weddings and parties, including my own where a guest said they were coming and didn't show up. NO CALL, NO SHOW. The couple or person of the hour took the time to include you on their guest list, mail out your invitation and most of all they wanted you to share their special day with them. The least you can do as a guest is to call them to let them know you can no longer attend.
Instead of making one simple phone call, you let the couple or person of the hour look at the empty seat or seats that could have been filled by someone else. I guess you forgot that money was spent for you to be sitting in that seat. Don't get me wrong, I know emergencies happen, but if it isn't one contact the couple, someone in the wedding party or a family member to relay the message.
If something comes up and you can't attend the next wedding or event you were invited to, call, e-mail, text, IM, BBM, Tweet, fax, or whatever form of communication you can use to let them know you can't attend. The couple, man or woman of the hour will greatly appreciate you.