The bracers are there so that you can pick fire bash shield rune for the aoe dmg and still have very good single target on rift gaurdians I believe
Not to sure with fire Shield Bash, the added effect from the fire rune only applies to monsters after they have been killed and it only fires in a straight cone if i am right.
The physical rune Shield Cross would most likely perform better in terms of AoE, the extra shields beside and behind you crit for about 100-200m if i remember well.
But pretty much the higher you go the more HP the monsters will have making it even harder to kill white trash in time before you start falling behind in the timer.
I love shield cross I really do, but it's far inferior in actual practice. Check out the video on manner cookies rolands build linked on main page to see high level play that displays the power of crumble rune.