I did a quick bit of spreadsheeting, (your to-hit chances are correct):
With a d10xd10 +10 attack
Against Def 6, it does an average of 15.88 damage per attack and 17.64 per hit with 5.56% of hits being crits.
Against Def 12, it does an average of 8.50 damage per attack and 18.89 per hit with 11.11% of hits being crits.
So, depending on how nasty the crits are, good armour is very useful in the long run, (even better than your first blush estimate), and may only marginally make the hits which do happen worse.
no subject
With a d10xd10 +10 attack
Against Def 6, it does an average of 15.88 damage per attack and 17.64 per hit with 5.56% of hits being crits.
Against Def 12, it does an average of 8.50 damage per attack and 18.89 per hit with 11.11% of hits being crits.
So, depending on how nasty the crits are, good armour is very useful in the long run, (even better than your first blush estimate), and may only marginally make the hits which do happen worse.