Still, it undermines the importance of the player's own character, the decisions you make on how to specialise them, etc. if you can technically have one of your companions a horse archer, another a berserker, a knight, and also an archer. Also, what would happen with commands now? Would everyone follow your companion's orders?
Not that I think it's a bad idea, just not quite in line with how I view the experience of playing the game ought to be. Were it a feature that switched the player's control to his closest companion should the player get knocked out in a battle, or an option to lead as one of your companions in the game if your character is unable to fight, then that might be a different matter.