Post 1 - There are times when you do still need people that put out a lot of damage.
Post 2 - There are a very limited selection of weapon/job combinations that satisfy that requirement and the gap between those weapons/jobs and the alternatives is very large, in my experience.
Post 3 - More explicitly stating that the gap between people that do a lot of damage and people that don't have the right job/weapon combinations is very large.
In summary, if you don't have a good weapon/job combination then you're so far back from the people that do have those things that you're effectively not a DD in high performance events when compared to them. I don't know why anyone cares about my choice of non-magian, non-R/M/E example weapons, because they're all so much worse than the ideal weapons that it's honestly not even worth considering performance increases from trading one for the other. If you're like the OP and you're asking a question about what the best DD is, then the honest answer is "You aren't going to be a really respectable DD until you have a good level 99 Mythic, Empyrean, or Relic." Please note that I keep bolding the word Good, because having a 99 Rhongo doesn't make you a DD. There are plenty of terribly balanced and utterly useless weapon options at the moment within those three categories.
I'd say these are the DD weapons for each job:
DRK - Ragnarok, (Liberator)
WAR - Ukonvasara, Conqueror, (Bravura, Ragnarok)
SAM - Kogarasamune, Masamune, Amanomurakumo, (Yoichinoyumi) <--- The first is much better than the last three, but all four work to a degree and excel against different targets
DRG - Ryunohige (40% better than Gungnir, which is better than Rhongo)
MNK - Verethragna, Spharai, (Glanzfaust) <--- With the qualification that monk damage isn't very strong against high enough level targets
If you don't have one of those yet and want to go DD to play around, grab a high level GS and do it. Don't invest in Borealis cells if you're building towards one of the above options. Please keep in mind that the preference may switch around (for instance, in the event of a Resolution or Shoha nerf), but it's unlikely that the unlisted 1H jobs will ever suddenly become competitive because of the way in-game damage calculations work. Also, keep in mind that some weapons do passable damage and serve a more utilitarian purpose (like Apocalypse), so they might be desirable for other reasons. That doesn't mean they'll suddenly do good damage in Mul, it just means that you might prefer being a mediocre DD that can solo on DRK.
Edited, Nov 26th 2012 5:32pm by Byrthnoth