(A) Because not everyone is a nightblade.
(B) It fills an empty soul gem - it's really for farming those.
(C) It returns resources other than health.
(D) It's a actually quite a good sustain skill for things like vMA, which I keep forgetting myself.
(E) Because it affects 2 targets.
(F) It used to knock nightblades out of cloak. Not sure it still does that, but it actually worked as a detection skill, used by classes other than nightblades. This part is/was probably unintended.
There. Is that enough for ye?