When being hit for hundreds or even a thousand health the little bit of defense that is upon heavy armor is no match at all agaist the damage being dealt. One also has to ask why there aren't any defense buffing spells in the game (though buffing next to no defense on equipment is still of little use).
there are defense buffing spells. privateer valor's shield, valor's armor, valor's fortress, that stuff. i believe buccaneers also get a defense boost spell, but i don't know. and armor isn't supposed to make you impervious, just a little boost. any damage reduction is good.