What does a DRK want in a party?

[ZDamned]

They're analogous, they override each other.

Mathematically, there is absolutely no reason for a warrior MT, or OT to have the ninja apply the slashing De-buff.

As an Off tank warrior, you should not be using Butchers Block anyway, as it's only 20 potency over 3 GCD's higher than Storms Eye.
So the warrior will sacrifice 20 potency but a ninja will sacrifice 60 potency in 1 GCD, while being capable of outputting more power per potency.

Basically, a Warrior refusing to do it, gimps the raid with a ninja present. Unless your Ninja is absolutely terrible, or 20~ ilvls below you.