Its serves no purpose other than to handicap assassins in the current meta, this itself makes no sense as assassins have no advantage over other heroes other than deflects, which are somewhat underwhelming.
A solution to this which would let assassins still hide their guard would be to remove the guard degradation and make it so that as long as they are holding the guard in a direction it stays.
The reflex gaurd hides your attack stance to making your attacks 'sneaky' to unobservant opponents. The issue i have with it however is that it really only works best on pc and not console due to the mouse being more responsive than a controller analogue. It's a double edged sword. Great for offense, terrible for defense (which is what an assassin is all about anyway)