I thought that too but it still didn't seem right. He was in a position where there was no getting out without fighting, the only reason he wasn't in the front lines was because the dwarfs moved their army in between the elf army and the orc army. But the elves were still stuck in between the orcs and the wall that Thorin made so unless the dwarfs stopped the orcs completely, the elf army was still going to have to fight and the elf-king should have noticed that. But even after he sent his men to attack I still didn't see any arrows. And we have seen in the past how good elves are with a bow, so I wouldn't think friendly fire was a problem. I will have to watch it again, but I don't think that a single arrow was fired throughout the whole war except for by Legolas when he was on the tower.