Why would you ever want to play a shooter with a controller when a keyboard and mouse is available?
Because nothing beats wiping the floor with someone who thinks they are leet and then telling them you did it using an Xbox controller.
Seriously though, mouse has superior anologue right?
So does an anologue stick over keyboard for movement, use combined Mouse and controller for best of both worlds.
Unfortunately Fallout NV and 3 have been screwed over by GFWL setup and you cant use both at same time, my only gripe about Fallout really.