Siphon Life - Why even hit this button?

I’ve been leveling a Lock alt and I just switched to leveling with a Destruction build using the Metamorphosis rune from leveling as Affliction. Part of this decision was driven by how truly awful Siphon Life is. This is in a spot on the tree where it should be a truly impactful ability, but I don’t understand the point of even hitting this button. It does so little damage and healing that it doesn’t feel like it’s even worth the GCD even if it didn’t cost any mana.

What is the point of this ability? When would it ever actually be worth casting? Why is it so pathetic?

It is laughably bad in SoD. The only use I can see is if you take Soul Siphon.

Should be boosted with a rune. Right now master channeler overshadows it.