It was explained in this thread by others. You came up with a good reasonable need on your own.
Well, not really. I just reviewed the thread (and discovered that I must have posted photos of my single jaw carrying asymmetric objects someplace else). There are only a couple of references to what I double jaw grapple on a root grapple can do differently. I don't have any trouble doing these things with a single grapple though.
1) grab around a bush or grab a tree limb. Doesn't count in my book. Single jaw grapples are narrow so it is actually easier to maneuver close to trees and bushes. I've never had trouble getting either one bush or leaving one bush alone while grappling other stuff. I can grab any limb I can reach (not a very safe way to trim trees though).
2) clamps harder. Not sure this is true but if it is it is irrelevant. The upper jaw in a root grapple just holds the material in place. The weight is all on the bottom tines. Based on the discussion it sounds like dual grapples clamp with exactly the same force anyway. (this is true for root grapples, a "clamshell" grapple operates on a different principle and by definition has only one upper jaw but the clamping force is the only thing holding the load, gravity is your enemy with a clamshell but your friend with a root grapple).
3) the taped finger confusion. You and KubotaKid like this analogy but as I noted, it is upside down. We don't clamp with our fingers, we clamp with our thumbs. Watch a baby learn to grasp. First they scoop and then they learn to use their thumb to pick up Cheerios etc. We are different from the apes because we have an opposable thumb not because our fingers move independently.
4) the most common reason given is the completely false notion that single upper grapples cannot clamp on irregular or assymmetric objects. That is just plain wrong. I'll post some photos below to show you what you can easily clamp in a single jaw grapple. I don't know where this myth comes form but it is flat out wrong. Why to professional excavator operators use a single grapple jaw?
5) construction debris. yes, this is a reason for double full length upper jaws but probably needs a bucket grapple or very tight bottom tines too. I consider this to be a special purpose item for skid steers and not something that is typically done with a CUT mounted grapple.
6) some folks say firewood. Well, I don't think of a grapple when I think of cut firewood. A bucket makes much more sense. Either two or one upper arm would have little effect on wood falling though the bottom. If you neatly stack it in the grapple then either one or two upper jaws would hold it in place as the weight is really on the lower tines anyway.
To reiterate, the advantages of a single upper jaw include lower cost, less weight and the "automatic" centering of weight. Double jaw grapples might have special uses but my point is that the vast majority of grapple functions are done easily with one. The reason most people have two upper jaws is that the dealer sold them a wide grapple designed originally for skid steers. Tractor dealers are notoriously bad at understanding how a CUT grapple is used and almost always over sell (heavier, wider, more expensive etc). Think about the demolition excavators that use single jaws. Those are very expensive pieces of equipment and they could put anything on the end of the boom they want. What do they use? Narrow single jaw grapples!
