As a Main tank and someone who's done both method, this helps tremendously in clearing the content.

Simply put, the biggest part of the fight is the add phase. Having 2 whole extra DPS helps tremendously here.

BTW, there's nothing stopping you from going as an OT with DPS in mind. Both PLD and WAR can put up some good numbers, and they can tank it in a pinch. We've had Titan die at the end in some runs and I just took it from there, eating the healing penalty until we cleared.