Since lighting at most locations for wedding ceremonies is poor for photography at best, flash use is limited, and the photographer is normally shooting from the back of the room, quick movements during the service may become blurred.  What better reason is there to hold the first kiss.  Remember during this and other special moments like the first dance, and cake cutting if you are in the background of the picture you will be there forever.  As a member of the wedding party, an honored guest, or as an official it is a good thing to remember.