My speculation is it's a real time chrono trigger type system, I don't think it's merely line of sight either I think it's just a way of getting across the tactical element. The enemies see both you then the mage but they focus on the mage, you then step out and use an ability that means they have to deal with you before they can get to the mage similar to the Sentinel class in FFXIII. According to the director they've focused the tactical elements similar to how sport teams would operate in which everyone has their own role and has to move accordingly to take control of a situation and take the other team down.
An example of this would mean that physical fighters will no doubt have to hold position or take the fight to the enemy to keep the fight away from magic users providing support while other classes do their bit as well.











