thanks for the link Jim, nice forum there

Heave and Pitch modes need sufficient rate to stop the car bottoming out over uniform bumps or the front or rear axles running out of travel as this would have the same effects as excessive resistance on a loaded wheel as detailed above. Specifically, we run the risk of a tyre(s) momentarily losing contact with the road leading to loss of grip. In simple terms, this loss of grip is part of what you feel when the car understeers or oversteers. Understeer being a relative loss of grip on the front while oversteer is the relative loss of grip in the rear.
Whiteline believes in using as little spring rate as absolutely possible for these reasons. We start from a small amount and increase, as we need it, not the other way round. As a full range suspension manufacturer, we believe in using the right component designed for each job to deal with its chief responsibility. That’s why we use swaybars and not springs to control body roll. We would no more design a swaybar to hold a car up in pitch or heave than we would try to get the spring to hold the body up in roll.
but, i see no 'reasons' there Jim, could you eludicate?

i understand that stiffer springs may bounce & lose contact with the road - but this is dependent on the quality of the damper is it not?

if springs push tyres onto the road, would not stiffer springs help this?

what is the disadvantage of using stiffer springs on the track? and on the road?