Well, the game is aiming for full open PvP. What distinguishes a friend from foe then? Maybe I
want to attack my "friend".
Anyone and everyone should be legit targets, therefore by definition "friendly fire" would be on.
In a tactical point of view it's great too. Casters have to watch out with their AoE spells, archers should pay attention to possible friendly targets near their targets. It is also a tactical choice to move in combat so that your foe stays between you and his archer friend, so the archer can't shoot at you without risking hitting his friend. Or maybe the archer wants to switch sides and shoot his former friend in the back
