Whilst the zoom effect during a Brutal Death is pretty awesome and certainly highlights the action, it could be a hindrance for the one dealing them as they do make your field of view smaller.
During these cinematics you simply can't always keep an eye on what the other opposing players are doing and where they are, making it harder to instantly continue your thirst for more blood (i.e. for multikill/killstreak purposes). This obviously wouldn't be a problem with the normal camera view.
By making it an optional setting you can decide for your own whether or not to have the zoom effect when you're the one dishing out the Brutal Death (the players on the receiving end should still get the effect regardless of their settings!).
This way, true pro's will distinguish themselves by cleverly turning this feature off.
