Steel defense is a buff warriors get. Specifically an aura that affects nearby party members to the tune of +20% defense. now what does +20% defense net exactly? About 3% Damage reduction. Compare it to the Crusader aura Steel Health. +20%HP(10% for others) and +10% HP healed. Completely obliterates Steel Defense. What about the Dark Knight Aura called Steel Spikes? 20% damage reflection, again completely blows steel defense out of the water. No contest at all, not even worth considering unless for some very strange reason you are rolling in a part with 2 crusaders or 2 dark knights.
I get the feeling steel defense was supposed to be +20% damage reduction which would make it competitive to the HP and reflect.