You aren't really noticing at the same time when all deceases/pets/autoattacks/procs are doing dmg to bosses, but overall everything adds up. Besides SR will be the best dump for unholy rune/death rune becouse it's dmg will top every other choice.
For PvP it'll have it's use becouse of necrotic strike mechanic - when you've enough NS stacked only way to remove SR is dispell and if it's on cd target is boned. Although i somewhat agree that there's not much difference between applying SR and see its being dispelled in 1 gcd, or applying NS and see its being outhealed in 1 gcd. Would like to see some light dispell protection added to ability, something like 25% haste for Xsec when it's dispelled.
So all in all i don't think it's a great ability, it'll see some uses but definetly a let down considering we got to see 1 new ability (outside of talents) this expansion. Lazy devs are lazy...