I can't speak for PLD cause they lose 2 whole GCDs for either stance but for DRK (and no DRK does not lose more from its stance dancing. We save a GCD and 1326 MP is very little for a DRK), Grit is treated more like an infinite Inner Beast that does no damage. When you consider the fact that Grit automatically reduces damage, and does not require healer intervention like Defiance (you don't take any less damage in Defiance) it kind of makes sense. I feel like DRK was designed with this in mind, using Grit as more of a cooldown than a stance. PLD does not have this design context however, so I have no clue what they were thinking there.