Each stance swap consumes a GCD & a set amount of MP that you'll barely use anyways. You need minimum 5 GCD in SwOath to see a slight DPS gain from swapping to SwOath & back to ShOath.

Grit while linked to GCD only resets GCD when you turn it back on, but because it will not allow you to drop until GCD is ready you lose at least half a GCD dropping it, more depending on latency. You need a minimum of 5 GCD to make up for the potency loss of turning Grit back on later. You need a variable amount for turning Grit off. You also run the risk of not having MP to turn it back on because all of DRKs high damage abilities requires 1700+MP, and some reduce MP returned.(Carve & Spit for potency) MP regen from Bloodweapon is also not a set amount. It depends on how many attacks you get off, and with bad luck or timing you may not even see enough to cover the cost of turning Grit back on.

Oaths have a much smoother transition than Grit. Anybody saying otherwise is delusional.