another thing to consider is how much DPS is lost holding onto a swift cast.
I forget the numbers, but going by pure guessing, id imagine its a 1 potency lost every second swift cast isnt used, when its ready.
(I dont have the game in front of me, nor do i remember most of the numbers atm, so ill update at a later time.)
but NOT casting a spell for 2.2s is the same as taking 2.2s off the timer for lucid. so it does save on SOME MP used (though not the full amount)

its still pretty bad though. there's not many attacks you can dok that actually do more DPS than the loss in DPS from e.reprise. (again, forget the numbers, but its something like 220 potency lost, after taking into consideration its base DPS added in. but maybe im remembering it wrong)