That's exactly it. A fight song/alma mater represents a whole school. A corps song represents just the corps itself. A drum corps exists wholly for itself.
Notice how it's the most popular and high scoring corps that seem to recycle source music. Out of curiosity, what did you think about the Glassmen, Blue Knights, Colts, and Boston Crusaders who all used mostly unused music in 2009.
Well retaining 100 members from last season is easier said than done. I'd say the corps I marched in was about half if not over half rookies and we make finals regularly.
The only corps I think might have been underrated is Phantom Regiment. From watching finals live, I thought they were superior to Blue Stars. All the other rankings I agree with completely.