This is discussed pretty heavily in the official forums, tank roles subforum.
WAR does ~5% more damage than PLD, when WAR has Defiance off and PLD is in Sword Oath stance. In other words, the difference is indeed negligible.
You don't take a tank in your party to do DPS anyways, so the discussion is largely moot. By the time you're at Titan HM the DPS are doing 200-300 DPS and at best with a tailwind the tank would be doing 100-130 DPS if he wasn't in his tanking stance, and nobody tanks out of tanking stance. In tanking stance you'd be lucky to break 100 DPS, I think I parsed at around 80 DPS once while tanking Ifrit HM where I literally do nothing but stand still and hit him.
I'm a level 50 WAR doing titan HM right now. With maim and storm's eye on(which is all the time if ur combo'ing right) my dmg combo is about 180-260-360, and if i have a good tank i'd be popping berserk often without worrying about aggro and do a 250-300s-400s with the occasional 700+ crit. In summary I'm not really sure it's only a 5 percent difference as you are saying. A WAR can out aggro a PLD easily if he tries to in defiance because of the difference in damage output and crit'ing often at full wrath stacks. Hell I even ran with my friends as a WAR dps for AK and it went fine. ( I have an ifrit's battle axe btw).
Additionally, when I was tanking Ifrit HM with defiance, I was dishing out above 100 easily, with heavy swing being at about 140-150 and butcher's block at the end of the 200 and criticals at 300-400.
So for all warriors out there, the difference is most definitely not negligible, and playing warriors is a blast. Even though people won't like the idea of you tanking titan( for good reasons unfortunately), you can tank almost everything else with no problems if you're not undergeared and know how to pop your CDs and use inner beast :3.