Quietus is weak because it was originally designed assuming a pre-SB-release version of Blood Price that still restored 480 MP on every incoming hit, and also 5 Blood for every incoming hit.

Had Blood Price made it to Stormblood's launch without getting nerfed into the ground, Quietus would have been usable every GCD or every second GCD while Blood Price was up, and your MP generation would have been so great that you would need to use Dark Arts on it, even though DA only adds 50 potency, just to keep from capping out, in between throwing about DAADs. The "weak" nature of the skill would have been counteracted by how often you actually got to use it.

Unfortunately, they didn't really go back to take a second look at Quietus after deciding to nerf Blood Price, and so what we have now is this weird, semi-undertuned situational skill where the potency doesn't really make sense, the Dark Arts effect really doesn't make sense, and there's this weird MP restore that's been tacked on in a post-release rush job that tries somewhat successfully to pull double duty and make up for its own shortcomings as well as Blood Price's.