I don't think we're referring to the same thing when we say ToAU. Treasures of Aht Urhgan? I frequently used WHM, BLM, SCH, BRD, PLD, WAR, DRG, MNK (any DD really), even NIN, and if people wanted to come on less optimal jobs I always let them, because it was always useful. And that goes for almost every event (Salvage was a bit restricting, but Nyzul, Einherjar, Assaults and even HNMs allowed most jobs). In Abyssea these jobs aren't useful. Don't get me wrong, you can still bring them, but there are just combinations that are a hell of a lot better, and it's solely because of weakness staggering. Because any DD can kill any mob in Abyssea in no time. But here's the thing, NQ mobs die in one WS, most HQ NMs still die in < 3min, farming low tier NMs still < 1min. The only time sink is staggering and farming, and since every job can farm just as well, staggering is the bottleneck. So why come DRG when you can come WAR and have almost all red procs? And having more people (i.e. DRG+WAR) doesn't help either, because fighting itself is the lowest time investment in Abyssea. Who cares if you kill the NM in 40s or in 1min? It's not gonna save you any noticeable amount of time, but instead may require another healer/supporter. Hence the uselessness of almost every job in Abyssea. In ToAU, more people equalled higher killspeed (where it actually mattered, when prolonged fights meant you would die easier), or easier fights. That's not the case in Abyssea, because more (and more diverse) jobs don't add to the overall group value.
Also, you don't need BLU, I never have a BLU in Abyssea, and rarely need MNK, and when I do I usually don't need NIN. Abyssea simply encourages small parties with certain jobs, you can trio almost everything in Abyssea very efficiently (including some Empyrean weapons) with WHM, BLM, WAR and possibly a THF for good measure.