Note that it gives 13 defense skill, not rating. The amount of rating required to get one point of skill varies with level, but the benefit of skill is constant. At level 80, it provides the equivalent of 64 defense rating.
likely was not added at beginning of WoTLK as death knights could dual weild tank weapons or use a 2h weapons with the rune of the stoneskin gargoyle for tank weapons. it is recommended that with the advent of this rune, tanks that are already past the defense cap use this rune with a slow melee weapon if they would be defence capped, to increase damage. at lower defense levels, it is advised to use dual tanking weapons to get two loads of defense