Higher armor ratings make guard more useful, since the damage dealt to you is lower. By the by, I just did some testing regarding the armor rating of shields. It seems to add to overall armor value, not just reducing damage from the front but from the back as well, at least to some degree.
A bit of background info: Level 16 Legionnaire, 71 Armor, 18 base Constitution + 16 from gear.
Comparing Shield of the Emperor (33 Armor) with Inquisition shield (5 armor).
Both with and without the temporary 200% armor bonus from upgraded War Cry.
I do have Turn the Blade (20% damage resist from the front). I don't have Bear Mauls the Wolves (immune to flanking).
No armor bonus passives.
For the record, these are rough averages, and the numbers were from the damage dealt by zone 1 melee Red Templars.
Routine
Shield of the Emperor:
1 from front, 15 from back
1 from front, 5 from back (War Cry+)
Inquisition Shield:
3 from front, 24 from back
1 from front, 11 from back (War Cry+)
Threatening
Shield of the Emperor:
25 from front, 115 from back
1 from front, 45 from back (War Cry+)
Inquisition Shield:
38 from front, 130 from back
1 from front, 60 from back (War Cry+)
Perilous
Shield of the Emperor:
76 from front, 303 from back
1 from front, 180 from back (War Cry+)
Inquisition Shield:
91 from front, 338 from back
16 from front, 210 from back (War Cry+)
Also, the duration of War Cry's bonus armor is broken. The buff lasts 10 seconds like the tooltip says, but you start taking regular damage again after 4, the default duration of War Cry. Come on, BioWare. Do they actually test this **** or just write it and hope it works?