A third pin adds to the labor of assembly.

As to the reason for using it, I might suggest that it looks more premium, and the examples I have seen seemed less flimsy than similar materials with only two pins (and logic dictates this would often be the case...the third pin adds structure and reduces flex in the scales).

I think it's like bolsters. It's there because it looks nice and it was a way to add profit for the superior makers that could produce it economically.