I don't really consider the backstab overall as adding or removing immersion considering the nature of the game as a whole; i.e it is immersive enough on its own.
Also, I like that the impact for the backstab is heightened and that the player feels visually and aurally rewarded for making it happen.

This was generally cut-back and not near as satisfying in the second game.
That said, I can understand your point of view, certainly, and from a pure logical game-play flow, it would make more sense for it to be shortened. I just like the sense of "time slowing down" and the deliciously visceral impact as it was delivered in the first game.
