Its a one-shot scenario, miss it and its gone. The preparation is extensive and the post-wedding work, particularly if you're shooting video as well as stills, is time-consuming and difficult.
I have enormous admiration for people who do this full-time and anyone who underestimates what they do or complains about the money that they charge has no idea of the work involved.
It can be very rewarding and often a quite moving experience to be part of the history of two families and to have the responsibility of recording social history for future generations. That responsibilty can be overwhelming at times and the relief when you realise that you have the images is enormous.
All images shot with Canon 7D.