I agree that comparing raw potency between different jobs is not definitive. But I'm using raw potency to compare two different attacks by the same job. DRG DoTs vs. DRG non-DoTs. MNK DoTs vs. MNK non-DoTs. Potency should be a pretty good indication of relative damage output on one character, since the Skill/Spell Speed (and other factors) on the same character on the same job should be the same.
However, I agree with your larger point. It's difficult to get raw damage numbers from potency between different jobs, especially if you don't look at the conditions surrounding the use of that skill.
For example, the OP suggests that Dragonfire Dive is much worse than BLM's Flare. I think this is silly.
Dragonfire Dive has a potency of 250 to all mobs in the area. Dragonfire Dive looks awesome, and it is awesome.
Flare has a potency of 260 to all mobs in the area. Flare looks awesome, and it is awesome. But Flare has a 4 second cast time (depending on Spell Speed/Astral Ice). And Flare uses all of the BLM's MP.
10 more potency doesn't sound like much. The actual damage numbers can be larger, depending on the circumstances.
But then again, imagine if Dragonfire Dive required you to stand in one place for 4 seconds before it worked. And imagine if it used all of the DRG's TP. I think that would be much worse.
That's why simply comparing DPS output is not really helpful. The circumstances are different.
DRG is helpful to the rest of the party in ways other than simply putting out large DPS numbers.
2 good Stuns, an amazing Slow, a piercing debuff that also helps BRD, ranged attacks, ability to close distance to ranged targets quickly, jump attacks that are variable depending on where you want to end up, large HP pool, ability to move while attacking, etc.
I don't think it's enough to say that DRG isn't putting up the highest DPS numbers, so they need a buff (and it hasn't been shown that they're putting up sub-par DPS numbers). I think you need to recognize that they're helpful to the party in other ways. That's what "utility" means. DRG's utility is not just its DPS number.