I only have one cel without the sketch, and I 
miss it dearly. I think sketches are wonderful 
because they are beautiful artwork just as the 
cels are! Of course, all you see are the cels on 
t.v. .. but there is whole lot more to production!
I also am thrilled to get layout drawings! And 
genga are beautiful as well (but i have none so 
far ;_; . 

Aside from what I have said, another great reason 
to get a sketch is because it reveals to you if 
it was a key cel or not. 

I say go for it if you really want it.. but if 
you can live without it...don't bid any further.