To Simon's points, I agree generally, but would point out that when I shot weddings, I tended to shoot two different types of photos.
The "money" shots - the ones critical to the event and those that are expected of an experienced professional; and
The grace note shots - the ones that record the nuances, the unexpected memorable moments, the moments in and around the oments of the day that aren't necessarily expected, but certainly form the fabric of the memories.
The latter type of shots are quite unstructured, and usually involve more shots, and a few more failures.
The photographer's work isn't really a success unless both types of shots are accomplished with some vision and flare.