dfwkeys.... you are wildin'.... LOL!!!
But yeah - in a marching band, be it "black", "white", or polka-dot, there are opportunities to enhance your musicianship. You're in frequent contact with other people who play the same instrument - some of whom may have lots more experience and you can learn from them. You may meet a director or section leader who is concerned with musicianship rather than over-playing or just dancin and puttin on a show. The music you play may open your mind/eyes/ears to new things you never thought you could play. And, hopefully, you'll be forced to practice your horn on a regular basis.
There are those risks you should always be aware of however, that have already been touched on, but I think the opportunities for improving are greater than those of not improving.
As a matter of fact, it was in my "black" high school marching band that I played my first major blues run.... it was a tricky lick in one of our songs, and I kept practicing it until I got it, and then I was like... HEEEEYYYY!.... that sounds TIGHT! ... and I incorporated it into my playing. That's when my soloing and improvising went to a whole new level.
- keynote