Here's an anecdote:
My son started playing clarinet in elementary school. He found that he had a love for classical music and he found that he was quite good as a clarinetist. He started marching, as a freshman, in his high school marching band and found that he loved that. Then, his Director took the kids to see a DCI show at a theater. At the end of his sophomore year he surprised everyone by deciding to switch to horn/mello. He told me his primary reason for switching was to march with the Blue Knights. His senior year of high school he became mello section leader and given the task of teaching two other members to play the mello. That's when he found out that he really liked teaching. Last fall, while in his freshman year in college...working for a music ed degree...he decided he might be good enough, after three years, to get an invite to the Blue Knights. He was good enough and he's touring with them right now.
So...what will my son have gained from his DCI experience this summer? I didn't read all the posts in this thread so I'm sure all of his benefits have been mentioned. But here are the most important, in my opinion:
Decisiveness. Setting a goal. Working for that goal. Attaining that goal. Knowing he "can do it if he tries" is, in my mind, the greatest benefit.
First-hand exposure to and immersion into the major league of marching. Some day he will be expected to provide a school with a marching band. This summer will teach him what marching is all about in ways he never learned in four years of high school. He will have "walked the walk".
Is it worth the money his mother and I had to scrape together to pay for this experience? Hell, yes.