I find it rather unnecessary.
We only have a certain number of buttons on the controller and most are taken up.
Just raise the controllers horizontal sensitivity.
What do you find unecessary...the mouse or the suggestion of a button?
Yes you can increase the horizontal sensativity to turn quicker,but that then makes you less precise when aiming for smaller things,because it becomes too sensative.
Just needs to be balanced. You can be alot more accurate with a mouse,than you can with a thumb stick.
Like i said,if they can let you use a keyboard why not a mouse....at least give people that option....just my view