Mastered Darkness is not worth it

From the Infernalist Ascendancy, Mastered Darkness is not worth it. It Doesn't give you an advantage for sacrifice your weapon slots. Why should you mantain in Demon Form with only 10 stacks of Demonflame?

I would like to rework it to do it more rewarding like...

Demon Form get 5 stacks of Demonflame per second
Demon Form degenerates your Life 20% slower

What do you guys think?