Many people here are missing the point of the plunge change. First of all, you will NOT dark arts more often than now. DA plunge is the same as DA syphon strike or soul eater, but instead of using it on one of those two it goes on the plunge. Same amount of dark arts.
Secondly, they didn't add this potency increase for the potency itself, but just to make up for the mp you lose on a a plunge dark arts for the enmity.
Tldr you have the same amount of dark arts with the same potency regarding plunge.
Dark passenger is a great change as well, although you still lose some damage compared to a dark arts due to slashing so I really only see DP being useful for 2 enemies or more or to get a bigger enmity boost when picking up adds.