If while in Shield Oath you've already finished Riot Blade when a period of no damage taken begins, you can, with no effective mitigation loss, drop Shield Oath in order to get an extra 108 potency. If you swap to Sword Oath at that time, you drop the combo, reducing that combo's efficiency from 307 pot/GCD to 127.
Edit: to be clear, you pop Sword Oath as soon as you've finished the combo. E.g. (in ShO) FB -> RB -> (Drop ShO) -> GB -> SwO -> FB -> SB -> RA, and so on.
You only drop the combo if you can get more out of an extra AA strike than the difference from cancelling (which requires that you know precisely the window you'll be in Sword Oath for). While in a safe / Sword Oath window, you only keep Shield Oath up for that combo's finish if you need the enmity. This has been the norm since lv40...
Again, the Vit/Str dual or swapped contribution may well already be covered by the upcoming tank changes. There's no point in only one ability in the game scaling in that manner when its base enmity is fine. What's not fine is that it doesn't scale with damage-increasing effects, whereas Unleash scales with Darkside and Overpower with Maim, Berserk, and effectively Internal Release and Unchained. Have Flash and Unleash scale with Spell Speed, even, so that your SS-heavy PLDs and DRKs don't keep trudging along with 2.5s AoEs while the WAR's spamming per 2.38.
Why should the job automatically be the better at 30 anyways? I'll admit, I'm of the classes-should-still-be-relevant-in-some-levelcap-content camp, but I feel like making jobs the obvious better, that much earlier, just removes the one brief period in which we're allowed to make decisions about our class outside of the particular cross-class skill used.
That said, at least from a damage standpoint, there is no stronger ability equippable to a PLD, especially with a quick weapon speed blade, than Sword Oath. +12-17% dps at all times beats any combination of offensive CDs... (Since we have no actual AoEs until CoS, I suppose specifying single-target is irrelevant, as well.)