Better aiming + aim assist = insta kill versus someone stroking their stick trying to align sights...
I could see it helping sniping a bit, but I wouldn't expect a huge improvement honestly. These devices have been around for years and never moved past a niche market. There's a difference between keyboard and mouse on PC and keyboard and mouse emulated into controller inputs on a system designed for controllers.
Plus Destiny runs at 30fps which further limits how useful mouse inputs will be.