At times, I feel funny having discussions about the value of multimodal composition because it seems like an old person's conversation. My students would immediately tell you that they compose all the time in different modalities, particularly outside of school. To bring these different modalities into the classroom can only enhance students' understanding of composition as a whole and, as a huge side benefit, make their non-academic composing better.
I am not done thinking through these points, but for now, let me say that I have caught up to many of my students by making my own video. This assignment was to capture the idea of multimodal composition in 60 seconds (no more, no less). We also could not have any audio synching with its original video. What took me hours probably takes many of my students no time at all ... the above point confirmed. Students are already composing this way, and by bringing it into the classroom, we can help them compose more powerfully in these modalities. This took me hours in part because I was learning iMovie, but it also took me this long because I was thinking carefully about my choices and arrangement. Our students can learn to think like this too.