Rathrak, The Poisonous Mind would have been worth trying purely for fun even as a Rogue, provided Poisons and Envenom still scaled from Spell Power, which they did in WotLK. These days, they scale with Attack Power and Mastery.
Blade of the Unmaker as a Main Hand Weapon is BiS, whereas Electrowing Dagger is BiS for your Off Hand Weapon. While Electrowing is a 1.4 speed Weapon, it is still BiS for both PvP and PvE, as the way Poisons work have changed since WotLK -- they proc much more often on fast weapons than they do slow ones. Aside from that, the bulk of your damage does not come from Mutilate in Cataclysm like it would have done in WotLK, so there is little reason to pick up two slow weapons.
I hope this answers your question. Hope you enjoy playing Assassination in both PvP/E. :3