Again, you compare the 333 version of the weapon to the 359 weapon. It stands to reason that he would have the heroic version http://www.wowhead.com/item=56382 vs http://www.wowhead.com/item=59492 . The "gains" you are mentioning are significantly different than every stat you have said. Heroic Seliza is better than regular Akirus. Stamina is more or less pointless, threat is pointless, the only thing he gains is expertise, which is only 75 more expertise than crit, which are valued as equal. The agility gained and stamina lost, compared to the attack power via the strength on the mace, do not compare to the dodge and crit (via agility) gained from Seliza.
In short, Heroic Seliza's Spear is, in fact, a superior tanking weapon than Akirus the Worm-Breaker.