Could this feature potentially have to do with accommodating different plate / negative sizes and / or different types of cameras? :confused:
With a multi-faceted hinge, I could potentially foresee an issue with shoving it back in the holder... might not always be so easy.
Also interesting the picture of the paper negative in the metal frame. Although the text about holders is quite thorough, one clear omission, is showing such a metal frame for holding the paper negatives, even though they are mentioned in the text.