As others have said -- it's a good situational weapon, but not something I'd take over someone else. Basically, take it if you were going to have to d/e it anyway, because it may become situationally useful.

As for its use, it's great for periods of time where you already have a threat lead and need additional mitigation. Prince's dual-wielding phase has already been mentioned. Basically, use a spelldamage weapon to gain a threat lead large enough that your DPS won't pull aggro when you switch to the king's defender, use it until the tough phase of the boss ends or you need to generate more threat, and swap back to your spelldamage weapon. Think of the weapon swap like a defensive trinket that's on-use reduces your threat generation and increases your mitigation.

It's also useful for fights where threat isn't an issue -- like tanking Netherspite's red beam.

Personally I would enchant it with defensive stats and do the swap with your normal spelldamage weapon. Don't enchant it with spelldamage, because you'd only be making a great tanking weapon slightly "meh". Get a REALLY good spelldamage weapon with +40 spelldamage, and a REALLY good mitigation weapon with a defensive enchant, and swap them as the situation requires. That way you have two great weapons, rather than one being just mediocre.