We already have many ways to gain armor; Strength, our gear, shields, ect. However, resistances are much more difficult to come by, and later in the game, are some of where we get hit the most.
Since intelligence is less than optimal for us and since Gems no longer give specific resistances when socketed, Nerves of steel seems like it would have been the ideal thing to give us Resistances with: Strength for armor, Vitality for resistances.
Not to mention that Making Nerves of steel give armor makes it compete directly against Tough as nails as a passive, since both do a very similiar thing.
I just think it would be much more elegant if they'd made this perk give resistances instead; Heck, it still would help against melee attacks due to Physical resistance.