In my opinion, as it is now, DRK has an unfair disadvantage compared to PLD and WAR when it comes to stance dancing.

WAR gets a free stance change, with a 10 second cooldown, whereas PLD has to sacrifice a global cooldown to swap along with a small mp cost, which is okay because MP is not as important for a PLD as it is for DRK.

Grit seems to cost about 20% of a DRK's mp pool, which is pretty huge considering the cost of Dark Arts and various other DRK moves.
But, beyond that, I just don't understand why Grit has to cost MP at all, and the same applies to Shield and Sword Oath, though PLD isn't as punished for using it frequently.
The fact that it costs a global cooldown is, in my opinion, enough of a cost, and in fair balance with Warrior. PLD and DRK get a more frequent swap opportunity, whereas Warrior doesn't have to give up a gcd but is locked in a certain stance for 10 seconds.

So, I'm wondering, why exactly do PLD and DRK have these mp costs to their stances?