Live axle is only superior in one application: on a smooth, flat surface where un-sprung mass is no detriment. Some off-road folks prefer it as well because it will move up and down with the tires...thus maintaining dynamic ground clearance. This is true, but it's partially offset by the increased un-sprung mass making it harder for the springs/shocks to maintain tire contact with the rough surface. IRS provides better ride quality and better traction in the real world (imperfect surfaces) because, while it is over-all a bit heavier than a live axle, a lot of the weight (mass) is sprung so the springs/shocks don't have to control as much mass and they can react more quickly. Additionally, each wheel/tire is free to react to it's own situation without affecting the other (or the car) as much, if at all.