Wedding guests with cameras is a sensitive subject. Personally I don't mind guests with cameras. They are usually doing their own thing, taking photos from their seats or of themselves during the party. What needs to be mentioned is when a guest is also a professional or enthusiast photographer and brings their professional equipment to your wedding when you've hired another photographer for the job. A guest should never interfere with or take over your photographer's job.
A wedding day is full of precious moments that cannot be recreated. Don't let those moments be lost by other people getting in the way of your photographer or videographer.
Most wedding guests know not to distract from your photographers job but if you know that a guest plans, or has been asked, to come along and take photos during the entire wedding day it's worth kindly asking them to leave the camera at home and join you as your guest. I have found that when another professional camera shows up at a wedding the person behind the camera feels they have a responsibility to photograph your wedding day and quite often they get in the way of your photographer. I speak from experience.
Don't let your wedding album have missed moments. Hire a wedding photographer you trust and let your guests be guests.