How would that work though? You're leveling your desynthising skills up, while the odds for materia are fixed. And what would even the point of it? To cheat crafters even more? There are already options for gear you dont need: Market board or your GC to turn them in.
They put the materia melder in for convenicen; desnyth is tied to your character - you cant request it of another player (while you really should though), why have an NPC for it?