A Paladin Tank needs 2 weapons, a spell damage one, and a mitigation one.

One good example is Prince. Say you use S2 Gavel, and Kings Defender.
For Prince Phase 1, you would use S2 Gavel to build your threat lead so that DPS can start sooner.

At phase 2, you want more mitigation than spell damage. Prince hits fasters, and does Sunders. You want to avoid getting Sunders to stack. This is where I switch to a mitigation weapon.

I usually continue using the mitigation weapon until 20%. Then I switch to spell damage as I usually spam Hammer of Wrath at that point in my rotation.