This point of view will probably be derided, but 
sometimes I've kept the douga/genga and sold the 
cel.  Sometimes I really like the detail, 
delicacy of line, whatever, of the sketch(es) and 
it just didn't "come through" on the cel. In fact 
I've seen some really mundane-looking cels that I 
bought at the beginning of this hobby, that I 
ended up not caring that I have, that have had 
quite pleasing sketches that I'm glad to have as 
an independent part of my collection.  

I really think, with the rise of CG shows, that 
our eyes are being "re-educated" to appreciate 
sketches on their own, and that some amount of 
splitting is bound to happen.  OTOOH, I can't see 
breaking up each of the roughest sketches 
(including the really raw layouts) into 
individual items, even though that's how some CG-
show items are already being offered through the 
professional venues.