A comment and one story:
I've never understood the concept of print resales being an important part of a wedding photographer's compensation. What does he charge per print for the sale of 4 extra 8x10's to be significant? If I were doing wedding photography I would charge (hansomely! for my time and three good albums - Bride and Groom, Parents and In-laws. And by the way, if you want the negatives, they're yours. Anyone else taking pictures would be fine with me, I've been paid and my medium format formals will be technically superior to anyone else's there.
Now as to the question, should you shoot at a wedding? If the Bride or Groom is a close friend or relative, I believe you have an obligation to. It's too big a day for the bride to depend on one person taking pictures.
One of my now wife's best friends was getting married, so I took a rangefinder with a 35mm lens and a small flash for fill to the wedding and reception. I only shot one 36 exp. roll of Kodak consumer print film, very unobtrusively and never got in the way of the professional photographer. I never had the roll developed, pretty much forgot about it, since I was shortly getting married myself and lost track of it.
Ten years later, my mother found the roll of film in a drawer and had it developed at the local chain store, just to see what it was. Considering the passage of time, the negs turned out very well. My wife and I mentioned this to the couple, in passing, when we saw them next and they exchaged a strange look... turns out the wedding photographer had had some disaster and could not provide any pictures. They cobbled together an album with shots from the immediate family, but forgot that I had taken any. My pictures were a significant addition to their album, to put it mildly.
The unhappy end to the story is that the groom was killed at the World Trade Center. I'm glad his wife has those pictures.