How many current members of the corps are or have been members of marching bands?
The activities are no longer separate. DCI has become the moral equivalent of the graduate level of marching band. For the most part it's where the hardcore high school/college band kids go to get their fix of excellence.
Drum corps is a different experience than marching band with different instrumentation, yes. But the upper echelon of marching bands, especially in high school, has the same staff, arrangers, designers, style of show, and directors as the DCI corps. I would argue that DCI is setting the standard and starting the trends as opposed to it being the other way around as so many chicken littles here claim. SCV does a show, 18000 high schools wanna do it too.
The guy who asked "what's the difference" has a point. Drum corps has not lost it's identity, it has merely evolved. What's wrong with drum corps being another part of the music education community as opposed to being the red-headed stepchild of the boy scouts?