Necrotic strike and mastery?

If someone can answer this for me would be appreciated:

Im under the understanding that unholy (in pvp and WM) you replace scourge strike with necrotic strike.

With necrotic strike being “plague-damage” …….and our mastery being:… “increase your shadow damage” ….

why would you ever use necrotic strike if it complete nullifies your mastery?

Plague damage is a combination of shadow and nature damage, and thus scales with mastery on damage portion of the ability.

You really only lose out on the burst portion of immediate damage on the wound and you cant necrotic without wounds on your target (but you can scourge strike without wounds).

which is better for pvp, necrotic aura or decomposing?

Necrotic Aura

decomp aura is good with other specs that also reduce hp…
like dk/destro lock… you can get a 30% less hp debuff on the target and 1 shot easier