So, excepting the praetorian and bastion, interception makes an ally wear your full plate for an attack? To tell you the truth I don't really like it. What if the sentinel is caught sleeping naked with his wife by an assasin. He can't protect her then at all. Perhaps you should make it a flat +4 or +6 bonus (considering everyone can wear a chain shirt) or his armor bonus (so +8 usually), whichever is higher to help in cases like that. Also his challenge has an unlimited range, so he could challenge whole armies
. I guess E6 Kratos dipped Sentinel.