I agree with Krusty, rudder pedals are vital. You don't get the precision needed with a keyboard.
The rudder is useful in several ways. Its needed for adjusting aim accurately. Its much easier than if you are using just aileron and elevators to aim.
I find it makes a useful airbrake as well. I can bang in full rudder with opposite aileron and cause an overshoot. Or if I am dropping on a lower opponent I can use the rudder to prevent overshooting myself.
When you are in a steep turn with the stall horn blaring you can use fine adjustments of the rudder to prevent one wing stalling and flipping you into a spin. Getting closer to the edge of a stall can be the difference between winning and losing the fight.
Sometimes though you want to stall one wing. In a scissors fight you can bang in rudder and aileron together to cause a snaproll, instantly rolling 90 or 180 degrees. You could probably do this with the keyboard, but you get a lot more control with the pedals, instantly correcting if you go to far.
If I lose the rudder on my F6F its pretty much mission over for me (one way or the other).