Oh, Kimstu, I wish I had had this advice from you then! Thank you, the “sorry, we’ll miss you (read: fuck off inconsiderate slobs. Oops, still sore about the cousins.)” is a great tip and I will remember it in the future.
Those dudes may had emergencies and called the host and let them know.
However, I agree that the proper thing to do would have been to send “regrets” in a timely manner.
And that’s how she words it, too. Miss Manners tells it like it is, man. 
DrDeth, I applaud your interest in finding reasons why people would do something like this, but the odds of all five couples having a “can’t possibly show up” emergency are slim to none. My guess would be one true emergency (babysitter got mumps at the last moment) and four louts.
Yes, that’s my guess too. 
Maybe two.
UPDATE:
The host has returned to the office and needless to say, he is a bit annoyed at the no shows. Not one called or left a message on his voice mail. Not one came up to him to apologize or offered any excuse for not attending. Oh, and only one left a gift in his office. (None had a gift sent from the registry either. I am nosey, I asked).
Sadly, this was a hard lesson to learn for him and a handy eye opener for anyone planning an event in the future.
I work at a modest sized research/educational institution (~2200 employees) and every year we hold a recognition event for all employees who will be celebrating their 5, 10, 15, 20, etc. year anniversary with the institution. Generally, about 300 employees qualify each year for recognition and printed invitations go out. They have the usual wording:
You and a guest are cordially invited to attend…
Please RSVP by…
Please indicate if you will be bringing a guest…
etc.
We’ve been doing this for over a decade and it never ceases to amaze the organizers of how many people do not return the RSVP card or who show up with multiple guests. Part of it has to do with cultural and class differences. Our employees range from janitors to full professors and include many foreign researchers and faculty. We realized that not everyone understood what RSVP meant or the niceties of proper invitation etiquette. We finally had to spell out everything very clearly. To wit:
You and one (adult) guest are invited to attend…
Please respond by returning the enclosed postage paid card by…
Please indicate if you will be bringing a guest…
etc.
We also send followup emails reminding everyone of the 1 adult guest rule and if you haven’t responded in the affirmative, then don’t come.
All this has helped a little but we still have clueless people show up without letting us know they are coming.
wankers
It really has come to this, hasn’t it? Sigh.