When I was playing an Epeiros campaign a few weeks back, it seemed to me the armoured elephants withstood the javelins better than ones without, but does the better defensive ability make up for the cost?... that is for you to decide. I did much damage and had a lot of enjoyment watching men fly when engaged with my armoured elephants but they are costly and if you need a decisive unit to turn the tide of battle, armoured elephants would be my choice.