Blockade is a background ability of any tank. It’s difficult to imagine a tank without this spell. Blockade gains extra chance to block attack and gains extra attack speed.
Don’t forget that attack speed also affects your global cooldown. So it’s great to have extra attack speed especially if you have many spamable abilities.
Immunity to knockdown will help you a lot in PvP.
Blockade also has a very nice combo that gains you magic shield.