This would have fucked up handling quirks. For example, with the roll-rate on one bar, the car may have neutral handling... then as the front rolls onto the next bar, the increase in front roll rate would cause the car to tend towards understeer. At which point the lateral force lessens to the point where its back on just the one bar, has neutral handling agian, the tires catch, which loads up the rear, kicks it up a bar, throwing the car into an oversteer situation.
It wouldn't smooth out handling at all unless you used progressive bars such that the transition onto the next bar was a seamless rate increase, at which point you've just invalidated the need for multiple bars, as you could just use a single progressive rate bar that ecompassed the roll rates you were looking for. And that still leaves you with the relative roll stiffness under/oversteer issues described above.
So what advantage is this chasing, exactly? Swaybar stiffness doesn't have much effect on driver comfort, hardly any compared to spring stiffness. If it's not improving handling and it's not improving driver comfort or confidence, what's the point?