I think the ruined armors are there sometimes to provide a little "atmosphere," like ancient armors that have been in the catacombs for many years, dropped by dead adventurers that were unfortunate enough to have venturered into dungeons alone and never made it out. Or sometimes in a battlefield filled with various weapons and armors, you may see a ruined armor, presumably because the victim was so slashed to pieces that his armor was full of holes and rendered unusable. I don't know their use beyond decoration though.
