There is no 95% even without grazing hits in end boss fights, I can't think of an end boss that does not throw other effects. My biggest bugbear with grazing hits is that it is no defense vs it. If they had it as a stacking debuf that melee mobs could impart I could easily live with that (though other debuffs would be good as well)
Eg melee based mob (including bosses) has an x percentage chance to throw a sunder, the hit lands with sunder effect imparting a grazing hit debuff, mobs can now graze for the duration of 1 minute timed debuff, in that time a second lands (can land on grazing) and the timer is reset and a level 2 graze debuff lands if on a setting harder than normal or on normal just resets timer. This can continue for up to 6 debuffs on elite for the 13 or above. This makes individual trash mobs easy to deal with but pull a heap of mobs and your ac drops massively in usefulness.
Give orange named a 2 min timer, red named 3 min and purple 5 min. This will mean that at the start of the fight an ac based toon will need less healing but will certainly scale as the fight progresses giving the option to swap tanks to give him time out and other relevant tactics.
I may actually repost this under a separate suggestion.