I think sketches are kind of like icing on the 
cake. For cel-based shows, the cel is going to be 
the most important part, because it's what's 
actually filmed, but I definitely have a soft 
spot for genga, douga, etc. They're useful for a 
glimpse into the creative process, and often 
times the roughest sketches towards the beginning 
have a lot more "soul" and "character" in them 
than the final sketches do.

That being said, I know there are cel dealers who 
take sketches, cut them up into pieces, and use 
them for scratch paper. -_- I'm sure that they 
wouldn't do that to good sketches that they could 
make money off of, but the point remains that 
sketches do get destroyed on purpose sometimes.

But, no. I wouldn't pass on buying a cel because 
the sketch was missing or damaged. I'd think it 
was a pity... but I don't buy cels for the 
sketches. The sketches just happen to be happy 
inclusions that I like running into. ^_^