I was just at a wedding this past Sunday as a guest, and during the first dance, I happened to be at the perfect spot for some nice photos. So I took some. No one else was standing there, so I didn't think much of it. A minute or so later, the paid photographer came over to take shots, but had to stand somewhere not so good (though close by). My initial thoughts were, "hey, you should have been paying better attention to the room so you were already positioned for this." Plus there were actually two professional photographers, so that neither one of them was where I was standing when the first dance started was kind of lame. That being said, I made sure I wasn't in the other photographer's way. I knew she had a job to do, and it would have been disrespectful of me to prevent her from getting the photos my friends paid her for. But that doesn't mean she gets first choice of every available shot. And since I was a guest, she couldn't tell me to move.
I think it boils down to proper planning and as best as possible, and making it clear to the B&G what the conditions are required for you, the professional photographer, to take the photos the B&G are expecting of you. I think most couples understand that it will be difficult for you to get poised nice shots if a papparazzi like crowd is following them, standing in front of you, and firing their flashes at inopportune times. By the same token, if you take the time to set up a particular shot (or shots) for photos that will ultimately become something that is your signature style, then I don't think it is appropriate for other shutterbugs to be in your way. Those shots are some of the most important deciding factors in choosing a wedding photographer, and someone else taking the shot is actually stealing your work IMO. I believe you are entitled to any and ALL photo sales when you carefully orchastrate photos.
If you see a movie production crew on the street filming a scene, and break out your little (or maybe fancy) video camera, and the filming crew sees you, I'll bet they'll rip your video camera away from you, and no one would think that was unusual.
I've actually seen in wedding contract clauses that prohibit other people from taking photos of poses they actively stage. I don't know if weddings are considered "public", but nonetheless, I would talk to the B&G in the planning stages about the conditions you work best in. If you don't mind lots of shutterbugs, then fine. If you do, then communicating this to the B&G (in advance) will give you the "authority", so the speak, to control the situation so you work at your highest capacity. Plus, it is likely the couple will communitcate this to the MOB, MOG, and all other key people, and this may create a situation where one of them will do a better job of controlling the rogue wannabe photographer than you ever will w/o coming across as a jerk.
For all we know, the ninja photographer was the girlfriend of a friend of a guest, who didn't want to go to a wedding solo. Wouldn't it have been aweful to miss great shots for someone so removed from the B&G?
Thanks for reading my long post. It's a slow day at work today. 
