well depending on how the different rates are achieved there doesn't have to be a transition points between the two rates...If 2 springs are used sure, but if one spring with a varying diameter is used you wont get that problem...
But why? Its much more for comfort, or to allow a vehichle to carry a large load with out having rediculous spring rates when its unloaded.
It doesn't help as much with body roll because at the point where the body is starting to roll the spring is weak, its not until after you have a good bit of body roll that you get the roll resistance you want.
My RC doesn't even have sway bars so a progressive or helper spring is even worse here...
Second it can be hard to tune. You only have one shock setting trying to work for a whole range of spring rates, so you could only set it to optimal at a given point during compression (I.E. not tuned as well as it could be until the car is at the exact speed in the exact radius of turn)
The same problem is even worse if you are trying to tune sway bars in conjunction to spring rate...
Progressive springs, in general are more of a comfort then performance thing...
Edit: that picture/concept is very cool as much as I have seen that, I never really though about it used to lower spring rate.