WAR gets hit significantly harder than just 20%. Look into this fancy thing called a Shield. Oh, and T13 spikes for P1/2 are tuned perfectly around PLD mitigation timers; WAR is going to be deficient.
And while an argument could be made that WAR MT is better in P4, the major drawback is that it is no longer safe to wear full Ruby, which is going to cut down WAR damage output considerably for the rest of the fight.