since adamantine and stubborn negates 1 dmg by doing nothing, just playing normally (you normally hit someone or dash, i hope) makes guard rlly weak, because you need to do a pose to guard. Since you "sacrifice" 1 action to guard, it is reasonable that it negate 2 dmg.

In the current state of guard, you need to run another damage reduction stone or vigor to not suffer punishment. if guard negates 2 dmg, ppl will to start to use usually and it can be run other stones instead of just negation shiftsones. And it will be a good shiftstone, instead of a mid shiftstone