This is an evergreen, ever returning question. Many may think - especially today's economical crisis - that if it is really necessary to hire a professional photographer when so many people have cameras? The guests and relatives can make so many photographs, do we really need this?...
Perhaps it is possible that a family friend who has experience in photography will be able to take nice shoots, but.... there is a but. One of the most significant difference between a professional wedding photographer and a hobbist, that the specialist thorought the years, under continuous development of himself and his equipment had already learned from his own or other's mistakes what are the things that must bring attention to and be captured, what are the most basic mistakes that photographers would do that he will never do again. These could be really simple things, but still a hobbist might forget about while they are disturbed by the flow of happenings.
A professional photographer's job to be there and focus on the event, while the guests should be guests... You can expect a professional to have a backup machine in case something goes wrong, but what about the hobby shooter? Can you expect a hobby shooter to spend thousands of pounds on cameras, lens, computers, software, flashguns, light technology for the best possible results? Some parts of the weddings you have very little time to capture the right moment. If the photographer does not know what is coming, if he can not find the right position, the proper light conditions, he will not be able to take a nice photo and the photogenic moment will be lost...