Unlike the real world, Mormons only date for a few months (3 is the average) before they get engaged. This leaves very little time for you to introduce your significant other to all of your friends and relatives before the big day, which means that most of the time those people won't even meet the love of your life until the wedding day (or close to). So Mormons send out wedding announcements with pictures so that we can know what the person looks like before the happy day.
On average, nonmembers date for a longer period of time (a few years...?), which gives their friends, families, and significant other time to meet one another. So when the proposal comes, the friends and family have met the significant other and they don't need a picture of them. OHH!
Does this make sense? Is it all coming together now?
While we are on the subject, I don't like kissing pictures. First of all, if I haven't met your fiance then I would like to see his or her face in the picture so I can know what they look like. Second, I know you kiss and thats great; however, I don't need to see it. Third, Since Mormons usually invite everyone they've ever known and everyone that their parents have ever know to their weddings, it is nice to have a picture where you can see both people properly (not too close up, not too far away, and not a profile shot) so everyone can know what YOU look like now (since it could be YEARS since you've seen each other).
Don't get me wrong, I love wedding announcements, I love putting them on my fridge, I love knowing where and when you are getting married... I just wrote this because it was on my mind (since I have been to at least 10 wedding receptions so far this summer).
P.S. My roommate hates it when you put where you are registered at on the announcement. It doesn't bother me very much but its something to think about.