I always use Necrotic against anything that can heal. Rogues can Recuperate, Warriors can... Bloodthirst? Other DKs can Death Strike. Warlocks, well, no need to explain. The only class that Necrotic is fairly useless against is Hunters, simply because they have no self heal other than Spirit Bond, which is a deep BM talent that 99% of PvP Hunters won't have.
As for specs, I'm not sure what's best. Not having Necrotic drop the Desecration is pretty huge. You'll have a hell of a time holding onto anything that can dispel your snare. You're also going to want Chillblains, which doesn't fit well into a PvE spec.
In my humble opinion, to try to minimize the DPS lost from talents, I might go with this as a dual-purpose spec:
http://wowtal.com/#k=ekcDfHSc.b0w.deathknight.
Endless Winter is nice, but you'll be swimming in RP as 2H Frost. So, it's your call man.