This is a vital distinction that separates the Assassin from Tempest and Artificer in that Assassin does not provide anything new to the conventional Rogue's arsenal. Put simply, it does not change the Rogue's standard metagame.
Hidden Blades is a ranged blade attack, not unlike Throwing Blades. Knockout Bomb is a ranged area of effect version of Knockout Power. The only unique power you get is Mark of Death which is very good but that's it really. Heck, the Assassin even has a passive that is rather similar to the one in the standard Rogue passives - Ambush in the Subterfuge tree is quite similar to Gaps in the Armor in the Assassin specialization.
However, there are no conventional Rogue abilities that can replace Flask of Fire, Flask of Frost, Flask of Lightning, Spike Trap, Elemental Mines and Fallback Plan. There are no Rogue passives that are similar to the passives in Artificer or Tempest either. They are all unique. You feel like you are actually learning something more.
While it is true that the Assassination specialization is not as flashy as the others, it does have some high points. Chief among them being the massive damage you can gain from stacking its passives/ability effects with, as you pointed out, other default abilities that are similar to it.
Some would consider the fact that the assassin skill tree only has "modified" skills and passives as a weakness, but it can be seen as a strength. Get passives from the majority of the default skill trees (particularly the right side of subterfuge, many from dual wielding and a few choice ones from the archery and stealth trees) and select a few choice active abilities that are similar to the ones in the assassin tree, and you can rack up so much damage. Plus certain status.