Corey Ann Balazowich is a professional Ohio photographer who thinks couples should strongly consider having "unplugged" weddings—i.e., ask guests not to bring cameras, or at least not take photos during key times during the ceremony. Why? Well! Guest photographers can ruin the images that the professional photographer—who you paid a lot of money to beautifully document your perfect day—is attempting to take.
Now, do you want to see some reasons why?
Reason #1: They just might decide that the first dance is the perfect opportunity to get a photo of the happy couple.
Reason #2: The flashes from their cameras blow out the photographer's shots—and there's no rescuing the pics after that happens. 
Reason #3: Corey Ann calls this the "red dot of doom." That focusing light beam is helpful when you take photos with your point-and-shoot, but it ruins other shots. (She says she could make this image black and white, but there would still be a faded out spot where the light beam shone.)
Reason #4: They can just plain get in the way. Plus, not only is this dude blocking the action for the photographer—he's getting in the way of a bunch of other guests who probably want to see what's going on. 
Corey Ann says that you can ask your guests to be photo-free in an elegant way, like with the officiant reading something along these lines: "Welcome, friends and family! Please, turn off your cell phones and put down your cameras. The photographer will capture how this moment looks—I encourage you all to capture how it feels with your hearts, without the distraction of technology."
Would you ever ask your guests to put down their cameras? Does an "unplugged wedding" appeal to you?