Because it diversifies what instruments you can use. Why should design be hindered? It's legal instrumentation so if it enhances your show, why ask the performer to play unnaturally to make it heard?
It's surprising to see people complaining about the activity shrinking and low attendance compared to other circuits. Maybe that's because DCA is far behind in allowing corps to design shows that fans want.
DCA already has a bad connotation with mainstream drum corps fans as being a dinosaur circuit.
As for amps "not being drum corps", should we go back to what some on this board consider what drum corps is? Rotary valve horns, symmetrical drill? Nothing wrong with that, but I guarantee you'll be playing for 10's of people and no one will join your corps.
You can't take some advancements while dismissing others. Asymmetrical drill, pits, Bb horns, non-tick judging, and amps have pushed the activity forward.