To hopefully help clear this up, as a member of a drum corps front ensemble I as an individual and the front ensemble have more of a job today than say 10 years ago to contribute to the overall package of a drum corps show. We don't just stand there and play because... that would be pretty boring, in all honesty, but we definitely do not dance to the music we are working with. We "pulse" which is used as a timing mechanism as well as a way to show the crowd that we are actually enjoying what we are doing. Because light travels faster than sound, we use pulsing and prepping (the up and down motion of the mallets dictated by the center marimba to tell the members of the pit when to play their assigned parts) as tools to keep time in the crazy listening environment that is modern drum corps. In my teaching, the center marimba watches the drum major, dictates the tempo and the rest of the ensemble watches to keep time, while everyone is also listening back to the drum line. In addition to pulsing, some front ensembles (see Cavies or Bluecoats) also add visuals into the playing to add to the effect of the show and some even go as far as almost acting (Phantom 2014/13) however that is not how I was taught so I do not know much about how all that works. Long story short, we pulse in order to keep time as an ensemble as well as to not look bland in front of all the movement and color that is happening behind us. Just a comment from someone who fell in love with that section of a drum corps and eventually was a member of that section!