I think a permanent cancellation of mana generation from outside sources would only end up being a nerf. It isn't a frequent thing, but there are times in fights where there is nothing to hit and so it becomes beneficial to turn off darkside and regenerate mana from supportive abilities. If you trait this you would effectively be giving up the ability to gain mana during phases of fights where there are no adds and the boss is out of range, leading to less mana lowering dark arts usage and therefore lowering dark knight dps overall.
Well it takes around 6 ticks to even compensate the cost of turning darkside on. Around 18s of total down time for just a neutral value, and then, 21s to have a minor benefit. (not totally sure about the exact value, let's call them empyreal)

Currently it's pretty rare, and marginal, and happens on some fights like Yojimbo and his 30s downtime which are a pain in the ***.