I think that it was their goal when they designed the numerous DPS. Sure, there are differences in their raw DPS output, but more importantly, you don't play a BLM like you play a BRD or a DRG. Too bad this wasn't the case with tanks.
For example, they could have made a tank that doesn't rely on combos...give WAR no damage reduction skill but more variations of Thrill Of Battle+Convalessence to fit with the massive HP pool theme, and, at least, making their tank stances all act differently, even if they bring the same eHP. If Defiance increased all healing received by 25%, it would be mathematically the same as ShO, but with a unique flavor.