Discussing modern safety requirements of heavy reenactment combat such as SCA and HMB, the topic of thumbs came up. Modern rules require the thumb be fixed to and supported by the backplate of the gauntlet, much like the thumbs on 14th century hourglass gauntlets, this is due to the risk of breaking, dislocating or crushing the thumb, or pretty much shoving it back into the arm (shudder).

However, ince the hourglass gauntlets went out of fashion, we see a boom in late 14th century thumbs being attatched by hinges or separate plates, and by the 15th century, nearly all plate gauntlets have the less safe, more technically challenging hinged thumb-piece.

The hinged thumb does offer larger plate coverage of the hand, however, it exposes an integral part of your grip to being literally bent out of shape or smushed.

So why is there such a plethora of a hard to make, structurally weak piece of protection for the thumb? What am I missing?

To support thumb grip maybe? There are a lot of longsword positions that require it. See link

http://www.hroarr.com/black-lance-5-finger-hema-gloves/