Just about every built-in microphone picks up the sound of fumbling fingers and camera noise (focus, zoom, tape drive). That being the case, you're always better off using an external sound source, be it a camera-mounted microphone, wireless mic(s), or line-level feed from a P.A. system.
As you may have figured out from throdown's reply, the number of cameras or audio recording devices you has determines your flexibility and eventually the finished audio. The more sources the better. For example, in two-camera ceremony shoots, I use an external mixer to blend stereo ambient sound with a wireless microphone. I feed the mixer audio to a stationary camera shooting wide from the back of the church. The stationary camera's audio serves as the master stereo audio track. The danger here is there's very little I can do in post-production to fix this file (garbage in = garbage out). This is a tricky approach because the church acoustics and wireless mic's distance from the camera can cause an echo if I'm not careful to fade out the ambient sound while bringing up the wireless mic signal (dialog).
Audio is still my main source of problems; while shooting my last wedding, an outdoor ceremony on a windy day, I left the groom's mic level up when he wasn't speaking. This resulted in my accidentally recording horrible wind noise. The only way for me to fix the wind noise was to chop off all the bass frequencies below 100 Hz during the wind gusts, which which pretty noticeable in the finished audio. (Luckily the wind died down for the last two-thirds of the ceremony.)