yeah well - depending on your build and the circumstances a 10ar os shield can make sense - I don't know sh_t about DoA, but if some do not have a armor boosting skill or the foes generate enough armor reduction on you the mod will still come to effect. insc you do not have the vs demon options etc, so for areas with a variety of mobs of one let's call it category such mod is easily on mind yet I do not think many ppl know the difference between insc and os concerning armor calculation. but it is on the wiki as well.
List of armor effects by category - Guild Wars Wiki (GWW)
see the difference between inherent and inscription. it is confirmed and actually true.