Well, it depends on what you mean "not used in the 
process". Drawing genga /IS/ the process of 
animation. Like Olivier said, there is not a genga 
for every cel (you are correct about that), but 
the genga are what are drawn to show the 
animation. These drawings done by the animators 
are cleaned up and in-betweened to make the art 
that we see on the screen.

For example: Miyazaki does a large number of genga 
drawings for his films (no, not all of them), but 
he does not do the douga or in-betweening 
(although he checks it all. Sometimes you can see 
his 'corrections').

In a way you can think of the genga as the 
framework of the animation that is shown, but 
another way to think of it is that the genga is 
the animation and the rest is just tedious 
monkey-work that you pay people in Korea to do 
because it is so boring that no one wants to do 
it. It is mostly tracing and making very minute 
changes to get from A-4 to A-10. There is little 
to no creativity involved, but /these/ are what 
are xeroxed off and painted into the cels. These 
are the douga.

So, when it comes down to it, if you are talking 
about "The process" as being only what is filmed 
and we actually see it on the screen, then no. 
Genga are not a part of that process. BUT, if you 
are talking about the process of animation, they 
are the very core of it. The rest of the process 
is just cleaning them up. At least, that's the way 
that Animators look at it. -_^
Ryo