The guests may have huge cameras with fat lenses.
Do they know how to compose the shot?
Do they know how to direct the couple?
Crowd control/PR skills?
Anticipating the moment?
Damage control if things go wrong?
Are they aware of customs and traditions?
Can they guide the couple who are less knowledgable?
Are they able to have one eye on the viewfinder, walk backwards, and know what's coming into the left and right of the frame at the same time?
Do they have the stamina to keep doing all of this the entire day?
Give any four-year old kid a D3x/1DsIII. I'm sure he or she would know how to press the button, too...