Mainly for mitigating Death Sentence. Warriors has access to Inner Beast. Inner Beast, on top of the extra HP from Defiance, and further 10% damage reduction debuff from Storm's Path, is a better damage mitigation tool against Death Sentence than just Paladin having Shield Oath and Rage of Halone.

Paladins have to rotate their cooldowns for Death Sentence. At one point they'll eventually run out of cooldowns, and will need a Virus from the SCH/SMN.

That's not to say Paladins can't MT however, my static uses a PLD MT for T5.