I haven't raided yet as i'm level 45 but I'd think that a Darkness spec assasin would be pivotal to a raid environment.
We can shield, lowering threat by 25% to a single target. Spike, or Electrocute add 9% damage from all sources if you go 3/3 Nerve Wracking. Whither decreases damage dealt by a massive 5% Dark charge's discharge decreases accuracy.
Sin's do a ton of Internal damage, which I hear is rarely resisted. And have Tons of survival tools. Again, I'm not yet a raider and might be talking out my ***, but a 9% Raid buff to damage roughly every 45 seconds seems like a hell of a reason to have a Darkness Assassin in your raid.