Jhill

I think it has to do more with matching items to compliment each other. Also sway bars do limit how independent the suspension is and in the example given by stillen hitting a bump mid turn (imperfect streets) could possibly cause a dangerous situation. Also the stiffer bars are going to put more load on the tires and prevent camber change so you need a more static neg camber and a soft tire compound to take advantage of the super stiff bar. If you have a softer bar it will have more roll but allow the wheel to travel and gain negative camber (a good thing) so now you don't need to start as staticly neg for the street saving you tires. In a sense when turning you are trying to absorb that energy and make it shift direction so your parts should match and compliment each other. If stiffest is always better than let's take the extreme, let's think how a go kart would handle (flattest turning possible, no movement there) and slap hard compound tires on there and watch it slide right off the track like a bar of soap.

Just my thoughts though.