There are too many things going on and too much to think about to try and synch the cameras at the start of the ceremony while on-site. We run all three (or four) cameras continuously (all in LP mode). I let the photographers or guests with their digital cameras, help me synch in post with the camera flashes.
In cases where there are no camera flashes, I use audio and eye-blinks to synch. Once it's synched - it's synched. Also, we're usually capturing four, six or even eight audio sources (six or eight if there are musicians, singers, etc.) and even with a digital recorder, we need all or most cameras for audio capture. That means if you stop recording, you lose audio. Also, we use backup audio for everything critical. So you see, we rarely have a camera that isn't capturing audio.
It would be so time-consuming to try and synch a dozen clips from each of three or four cameras! Tape is cheap. Harddrive space is cheap. Time is expensive.
Also, while you're camera is in "stand-by" is when the bride's dad goes to wipe a tear, etc. and you miss those once-in-a-lifetime little moments - those moments that only YOU, the wedding videographer, can give your bride. What is the cost of losing such a shot?
Your next question is going to be about LP mode, isn't it? I can feel it coming. Let's not hijack this thread any further - that should be a new post.