If you want to deal damage, play a DPS. If you want to tank, play a tank. You can't have the best of both worlds, because otherwise it completely negates the point of having one or the other - if a tank can deal DPS level damage, why take a DPS in at all? Just go 3 tanks, 1 healer, and call it a day.
Also to point out that Sword Oath vs. Cleric Stance argument. Cleric Stance always ends up dealing more damage, because it goes off the MND stat (healer's primary stat). When Sword Oath swaps out STR and VIT, then you might have an argument. Regardless, on my i90 PLD, using a combo'ed Rage of Halone on Sword Oath without Fight or Flight, I am hitting about 300 damage or so. On my i79 DRG, a non-crit combo'ed Full Thrust usually hits for about 800 - 900 damage. So it is still at least 1/3rd of a DPS' damage. Also, keep in mind that my PLD has about 350 STR, and my DRG has about 450 STR, so I cannot account for if they both had equal STR.
Don't worry about the numbers too much, know that if you didn't help DPS, you just might not be able to pass that DPS check. I cleared Twintania yesterday, with about 1 minute or so to go before we hit the enrage timer. Your contribution could mean the difference between seeing an extra Dreadknight, or an extra Conflag. Try going to a fight like Twintania where the off-tank does nothing but helping DPS for most of the fight, and compare the results if you were to help DPS, and you were to do nothing.
Or you can just play a Warrior/Marauder using DPS accessories - Marauder is better for having more cross class skills, given if you want to do DPS, you don't need Defiance, and all the other Warrior/Defiance-related skills anyway. I think I've heard numbers where a Marauder with Raging Strikes was critical striking Titan Extreme using Butcher's Block for 2000 damage (don't quote me on this).
Also: http://forum.square-enix.com/ffxiv/t...=1#post1868212
According to the link above, the poster's Inner Beast crits for 1.3k, and he main tanks everything in the game bar Ifrit Extreme and BCoB Turn 5 using pure STR build, and i90 DPS accessories.
Sounds like you're playing the wrong class, you should be playing a Marauder/Warrior.