Like when you pilot an helicopter?
I can understand wanting to move your head when walking or driving a car/boat, but in a plane it make it much more difficult to fly: You have to move the plane with the keys while trying to fire and keep your view in front of the plane with your mouse...
Gamepad is much more fun and feels natural, tricky at first for the mo-key brigade but you get the hang of it quick enough if you persevere and the fun is way more rewarding. I stuck with mo-key for years but once I gave it up I realised the challenge and enjoyment was much higher and even more for FC5 because of the different vehicles.