I am a fan of keeping things together, for the 
sake of a 'whole'. It really thrills me to get a 
set that has the background, all sketches and 
layout stuff. And, there is often times 
information on the sketch that isn't in the cel. 
I like to consider them a pair and I never 
separate the ones I have from their pair. But, 
that is just my opinion, of course.