well, i think having an unstuck sketch is more 
desirable than the latter.  You can mix and 
match bkgrds to the cel.

In regards to its value, i guess you could say 
an unstuck cel would be more valuable. Being 
stuck tends to have the potential for paint to 
peel off.  To me, it basically says the cel has 
this one hinderance, i can't add my own bkgrd.  
But if it really mattered, i guess you could cut 
whatever was attatched. 

A cels retail value basically is the amount 
people are willing to pay for it.  Sometimes 
people will pay the same amount for a cel 
regardless if it's stuck to a douga or bkgrd.