Inter-leaf friction in leaf springs causes slow compliance and reduces the damper valving needed due to their own internal friction acting as a damping agent. Hum, sounds like the perfect pavement trailer spring - no dampers required if the internal friction is high enough compared to the spring rate.
By reading TeriAnn's page I now understand what the differences btwn the "parabolic" leaves and std leaves are. The term "parabolic" may or may not be mis-applied. It would be pretty hard to visually see any difference btwn a parabolically arched leaf and a semi-elliptically arched spring. Which was a point of confusion for me.
These "parabolic" features are also found in the late model 65" GM rear springs. These springs feature the leaf tip pads, which is not all that uncommon, but they also feature a shim/spacer btwn each leaf pair at the center pin that creates an air gap btwn the leaves. So the leaves only touch at the tips, considerably reducing the spring's internal friction. I have found that Bilstein's std leaf spring valve stack is not enough with these springs.
I suspect that this lower overall damping value is what leads to complaints of instability etc.
I think it would be an interesting experiment to install such shims in a "normal" spring pack and see what the vehicle's behavior differences are.