Personally, I like having matching sketches.  If 
a cel is nice enough, I dont really care, but 
usually I think its neat to have all parts, even 
the douga.  I recently got a matching lost douga 
for one of my cels from Chaos ^_^, and I think it 
adds to the value of the cel if you have all 
missing pieces.  But, it's just one of those off 
hand things.  If you dont have the matching 
stuff, it's still fine.  Most people like to have 
the sketches just for themselves, sort of a 
potatoe-patatoe thing.  I say if you really like 
those sketches, and you have the cel that they 
belong with, then go for it!  I think it ups the 
value of the cel.