The Hammaborg Rain Guard Theory

The Hammaborg historic combat study group has proposed that what other historians call rain guards were not made for that purpose, but as a protective device for the hand.

I see three serious objections to the theory. The first is that they demonstrate the theory with blunt swords, showing how the leather could stop or deflect a blade sliding along your blade from the bind that would otherwise hit your thumb. A sharp sword in the same situation would meet the light leather edge on and probably cut through it to slice your thumb.

Secondly, some surviving rain guards don't extend very far beyond the cross, severely limiting their protective benefit.

Third, if the intent was to provide extra hand protection, there were better ways to do the job. A flap or plate that was concave towards the hand, as shown in this 1460 illumination, would provide better protection. So would side rings to the cross.