i believe the answer to that question would be they (larian) wanted you to have control over at least one of the characters at all times, thus allowing for player interaction in opening doors, chests, initiating dialog, etc.
personally, i'd have liked the option to control (or not) both as well.
my suggestion: (this assumes you have one) leave the ranged attack character (magic user and/or bow user) set to aggressive non-leader status. this will allow them to attack at distance before the bad guys even appear out of the fog-of-war and continue to provide cover while you loot chests with the leader.
