The rectangular shape adds some weight, but probably saves some labor costs. It also maintains the multi-use function for the JRB...poncho, flat quilt for indoor or outdoor use, etc.
I've never had my feet or shoulders off the JRB underquilt in an HH...it goes all the way to the side tie-outs, and stays high on the hammock all the way above my head and below my feet. I don't know if that's the shape or a size difference...the JRB is 78"x48".
Tightening the end drawstrings all the way will reduce the coverage, and can cause air gaps near the ends. Loosening them a little bit creates a better fit. Loosening them all the way creates ventilation that makes it more comfy between 55-75F (Jack calls this "opening the windows").
My homemade quilts are basically synthetic versions of the JRB ones. Not as convenient or multi-use, though.