Paladins already lose half of their oath gauge on swap and dump a not too small amount of mana. DRK have to spend mana to turn stances back on and have to spend large amounts of mana on the skill that is holding the class together, TBN, every 15 seconds.

Oh and you WARs already do more DPS then DRKs, have more mitigation then DRKs, and honestly not that much less utility then DRKs. The stance dance penalty is probably a carry over from when they were designing paladin cause its very lazy.