I mean, they're expensive (mostly, though I do consider my prices very reasonable, of course).
Well, think about it; how much are you spending on the dress, bridesmaid dresses, groom and groomsmen's outfits, your venue, the cake, car(s) and all the other little things that make your day special (but still add up to a small fortune)? What will you have to show for all this investment when the big day is over and done?
Memories fade, but photographs are tangible and forever.
Why risk not having your day captured the way you want it? Now I'm not saying that an amateur photographer can't take perfectly beautiful pictures, but there really is an art to being a wedding photographer that takes time to learn. We have to get all the pictures you want in the least intrusive way - after all, you don't want to spend your entire day posing for the camera, and neither do your guests. We also have to be able to organise large groups of people and arrange them in a way that flatters everybody - it's not as easy as it sounds!
A really good Wedding Photographer will pick up on all the little details that make your day special, and capture moments that you might miss (since you'll be busy being beautiful social butterflies for most of the day), and can also be a pair of helping hands in a crisis (trust me, I know; some of the things I have done go above and beyond in the name of a perfect wedding day)!
But basically, what I'm saying, is why take the risk? You 'might' get gorgeous photos from a friend of a friend, or a relative, but then again you might not, and you only get one shot at it, so to speak.
If your amateur really wants to be involved and gain experience - send them my way! I'm happy to have an assistant and help out a budding photographer - and then everyone wins! You get extra photos for free and I get an assistant, not someone who's blocking my shot!