I have to admit, I feel for both sides. The pro on his/her job, and the friend/relative wanting to capture the best moments. I'm not a pro but I shot my first wedding this Saturday. The situation was such: the wedding party's hired/committed photographer canceled a week prior leaving the bride and groom in a bind. They asked my cousin if she could shoot as a favor. My cousin readily agreed and said she'll do it as her wedding gift (which was very generous, I thought). My cousin doesn't have wedding photography experience nor did she have the enough gear so she asked me for help, advice and equipment.
I was willing to give her tips (again, I'm just an amateur), loan her equipment but I was really concerned that she might not produce the "great" (suitable for framing) photos everybody usually expects from the photographer so I said I'll shoot the wedding with her just so we can double our chances.
Needless to say, it was very difficult. Thankfully, I read through all your posts and advice on wedding shoots and consulted many books and albums on how to shoot a wedding. But yes, many of the guests did block our paths (walking the aisle, cake cutting, etc.) and us noobs obviously didn't have hands-on experience on proper positioning and jockeying for the primo spots. Also when doing group shots, many of the subjects were looking at the other cameras (not ours), so we have group photos where all the eyes are askew. We quickly learned to bark instructions and do hand movements to direct them where they should be looking at. I would say guests with other cameras do create a distraction and are obstacles for the designated photographer.
However, I also feel that since my cousin and I did this as a favor, we also feel that as non-pros we also did a very good job in producing the shots, and had we been guests with SLRs, we probably would've made a good showing shooting from our seats (except of course we'd be annoying to other guests).
In the end, the bride and groom were very happy to receive very great photos of themselves at no cost to them. My cousin was happy to have come through and her wedding gift was much appreciated. And I was happy that my cousin was able to deliver what was promised. My reward? Chicken or Beef.
P.S. I refused to do any post-processing and said that somebody else can do the post-processing.