Originally Posted by
ZeroEdgeir
The purpose of this change isn't to give you something new (yet... that is coming, give them time, you aren't the only class lacking in the 85-90 department).
The purpose of this change is to stop facerollers who don't know how to play their class, from speccing Assassination and continuing to use Backstab when Mutilate is superior. This forces you to play the class AS INTENDED. It helps eliminate balance issues, when the intent of the class/spec is more straight-forward, eliminates cross-tree issues of balancing damage output and such.
They wanted Backstab to be the main skill for Subtlety, while Assassination has Mutilate, and Combat has Sinister Strike. Blizzard obviously LIKED the talent-basis behind Assassination's usage of Backstab on low-health targets, so gave them it back again, as an execute-style skill, for the simple fact of making sure that it cannot, in any way, shape, or form, cause any balance issues with damage with Subtlety, because they use separate abilities now. ALOT of classes are being done this way now.
Example:
Mages: All start with Frostfire Bolt, at level 10, with their spec, get Arcane Blast/Fireball/Frostbolt.
It's to eliminate cross-spec contamination and balance issues.