If you haven't heard about it yet, the Conn/Selmer Company has come out with a trombone completely made of plastic called the "Pbone" that sells for around $160. They come in yellow, red, green, and blue.
Comparing Pbone to a brass trombone Hint: First time click "skip to playing" and then close your eyes. Listen a second time watching which trombone is being played.
Pbone quartet-How High the Moon
The real question seems to not be "does material matter?", but "how much does material matter?" I think this would make a great instrument for trombonists in marching bands and pep bands who don't want to endanger their main instrument in those environments. They come in most school colors to boot!