Look what I started.
If I can sum up what I got so far:
If the plow takes a blow that puts a outside shock load on the ram you are smart to put some protection in place to keep the that shock from being transferred into the hydraulic system in the form of high fluid pressure. It makes sense that if you hit the plow high on the retracted side you can't do damage to the hydraulic system because the cylinder has no room to move. Of course expect damage to the FEL or tractor. Something has to give if the plow doesn't trip.
I understand the point about the volume difference on each side of a DA cylinder but I think that is why Prince offers it as a relief/cushion valve. I would have to find out from prince but my guess is that it would cushion the cylinder by expelling fluid from the system........but I'm guessing there. I only say that because unlike air fluid has very little compressibility.
I am as well just plowing two blacktop driveways without higher obstructions but $60 some odd dollars is cheap insurance. If I was only in danger of hurting the cylinder I wouldn't care but I really don't want to do damage to any of my factory spool valves. I'm sure JD would have no mercy on the bill when they replaced them.