It was determined a long long LONG time ago that 3xDD + 2x support + 1x healer was the optimal setup as 4x DD 1xsupport 1x healer is simply too much of a burden on the healer / support and thus forces people to play defensibly.
False argument. You failed to define the 5/6 nor what the required roles were of the 6th spot, and thus you can dictate a false argument with whatever rules / conclusions you want. That is dishonest.It's hardly a trap. When your party is 5/6 people and it's time to fill that sixth spot with damage, it's not the time to be looking for emergency cures, a Dia your healer or 'lazy bard' could throw up, or even a secondary haster.
A RDM/NIN will produce more total damage output then a BRD/WHM, this isn't even up for debate. The first BRD is already singing march x 2, this leaves the 2nd BRD to sing Attack songs and refresh the healer. Dia III is 15% defense down, or 17.6% attack up. At 500 attack every DD would receive the equivalent of 88 attack, at 600 attack they would receive 105.6 and at 700 attack (now) they would receive 123.2 attack. This combined with the 80% melee damage easily surpassed double attack buffs to three DD's. Don't even attempt to argue this, it's already been demonstrated both on paper and in practice. The only jobs that could do similar were COR (buffs + ranged shots) and DNC (haste samba + melee damage + curing). A melee BRD/NIN could technically do similar, there was actually a small following amongst the BRD's to do this because they saw the same opportunity we did.
Anyhow as I've said before, this is all merit parties at 75, which don't exist anymore and thus the above points are moot. If we ~really~ wanted to get technical a RDM now can do something nobody else could, and that's provide a 17.6% attack buff to every DD in the alliance. I do this all the time on abyssal super boss's and tier III / IV VWNMs. This is where I was really hoping SE would go with, provide us with self-buffs that would enable us to lower the targets Def / VIT / Agi / Defense / Magic Defense / Magic Evasion and so forth. In this way we would enable the entire alliance to perform better, while also dealing damage.