WAR is definitely better at going in and out of tank stance since it costs them 0 GCDs, PLD basically has to make a decision on whether or not they want to be in Sword or Shield for an extended period of time which makes things a lot rougher. With gear, a lot of content (pretty much all) can be tanked in Sword alone, so swapping back and forth isn't actually that necessary. If you're forced to stay in Shield for some reason, you're in a situation where Oath swapping isn't ideal to begin with. If you're Hallowed and there's no mechanic damage coming out after Hallowed fades, then you're fine to swap to Sword there for awhile, but otherwise it's not entirely worth it simply because you may end up killing yourself or you may end up costing your healers DPS. Staying in Sword has them maximizing their DPS/healing for that Oath, whereas dancing around has them unsure when they'll need to prepare for more or less damage. I personally just pick one Oath and stick to it unless I'm going from MT -> OT and I'm in Shield, obviously then I'll take the GCD to swap to Sword.