I think most people have covered it well. I decided to go to a second body so I can begin the process of starting my own business in photography. And that will be next week when I receive my second body. I have friends who have asked me to shoot engagement, party and even possibly their wedding shots. There is no way I am going into any of those shoots with a single body. If that body fails, I am up the proverbial creek at that point.
Once you got your first wedding and it was successful, your next step should be to get a back up body for the ability to not have to change lenses on the fly and to also have a back up in case of failure.
While I won't be a full time pro anytime soon, the fact that I am getting a second body allows me to consider going into making money with the hobby I love.