Well, they can make it a dynamic mechanic.
At the beginning of your character, you get a free "feat" or skill which purpose is to give you "good aim" between your friends and foes, allowing you to not hit your friends with your damaging attacks, or heal your foes with your cure/heal effects.
And maybe down the illusion's wizard tree, or mischief rouge (Making up names just to point out the intent of the build) can use ability's to reduces this value, maybe trowing sand in your eyes or making you see more than one nonexistence enemy.
And with this effects in mind people can "miss" hit their foes and damage their friends.
Just balance the value so is rare to have skills or ability's that reduce or penalize your ability to have good aim, it will be a pain if every boss/character can make you miss and hit another target. This can make up amazing battle story's.
Sorry for the bad English.