The Assassination tree, home to the deep dagger Rogue, is a major part of Rogue specifications for both PvP and PvE play styles. While the Combat tree is focused on increasing white damage overall, and Subtlety about survivability and burst, Assassination is all about the liberal application of consistent damage through a repertoire spells and damage application modifiers (a la poison). Burning Crusade talents and spells like Cold Blood, Lethality and (Improved Poisons assisted) Envenom are amazing non-positional damage sources, with Mutilate being the positional coup de grâce to a fight.
All that said, it should be noted that while Assassination Rogues do benefit from three times the combo-point generation potential, white damage is reduced in comparison to other builds, most mobs in end-game are poison immune (and in situations where you need to change targets, you have to build back up your poison stack on them) and Find Weakness does not apply to white damage.
Knowing all this, what's new that might make wayward-but-dagger-longing Rogues want to go deep into the tree?