The crystal-clock in your camera is probably MORE accurate than the Arduino resonator-clock. 
But nothing is perfect, and if you shoot with multiple cameras they will drift apart after several minutes (or maybe an hour) of continuous shooting. And if the audio is recorded separately, you can start to see "lip-sync" problems after a period of time.
With professional audio/video they use a very-accurate master clock linked to all of the equipment to keep multiple cameras and multiple audio recorders in sync.
And of course, the playback system is equally important... If you have two copies of a DVD and two DVD players, and if there were a way to start both players at exactly the same time, but the end of the movie, they would probably be out-of-sync by at least a couple of frames.