Not calling anime a perfect example, but I know of scenes in animes where 6-8 seconds of filmtime were used up with only one drawing, one background, and panning. Likewise, if there is a moment of 'thinking' or non-action, you can shoot one cel as many times as you need to. Replacing the 'dupe' cel with a 'tween' will make the action smoother.

If someone makes a fast movement, shooting on twos make make it look like their arm jumped from one side of the screen to the other. Something else to know about shooting on twos is that it makes it very easy to smooth out an animation because you can just replace one of the duplicated cels with a 'tween' cell. I don't know why I would think it was him, but someone showed us how the films were placed on the pegs, and they clicked the camera twice before switching cels. It was a LONG time ago, man, so bear with me. I do remember taking a tour of Hanna Barbera when I was a kid, and I got to meet Robert 'Tiger' West, who was the Xerographer before Star Wirth. That means that only twelve pictures are drawn for each second, and each picture shot twice, so it is recorded onto two successive frames of film. What I keep hearing is that US cartoons are animated at 24 FPS, BUT they are usually shot on twos.

Pencil animation moving one frame