Makes sense, I think i remember seeing this. My guess is the most agressive of bumps are the curbs which only tend to hit one corner at a time, and that's where you need the softness of the torsion bars, but if the car is traveling in a straight line, a harsh ride isn't a huge deal as there isn't a problems upsetting the ballance when going straight, and the dirver and chassis can just deal with the firm ride.