You do get dedicated button controllers with even more buttons that the mapped gamepad, but that's an extra expense and I can obviously use it as a normal gamepad for other games that don't need a mouse. The rumblepad also as a wireless pad is more moveable / flexible that a full keyboard, your lap is good enough as a desk top.
Flick up to jump, flick down to crouch and so on. The logitech profiler software allows you to change the analog sticks into 4-way switches. The gamepad gives a more tactile response, buttons are spaced out in 3D, the WSAD movement is on a D-pad nd so on. Why not just use a keyboard? Well that may be ok for game nuts, but for me using a key board for games is hopeless as I spend most of the time trying to find the right button or pressing the wrong button.
You have to have a surface to lay the pad on to use it with just one hand, its buttons acting as a replacement keyboard. The Rumblepad is used with just the left hand, with the right still using the mouse in its usual duties. However I do employ the same method I have done with other FPS games. OK while FO3 can be played entirely with a game controller, especially if exclusively using VATS, as I'm playing as a role-playing first person shooter (RPS), that's not going to be good enough.
