I happen to like my sketches just as much-- 
sometimes even more-- than the cels themselves. 
They seem to have much cleaner lines, and it can 
be easier to see details that get lost in the 
paint and the colors. And sometimes, it's just 
amusing, to see all the different colors they use 
to indicate highlights, shadow, etc. :o)

But it all boils down to personal taste. If 
you're bidding on something, whether it's a 
sketch or a cel, and the price range gets to 
something uncomfortable, let it drop. You need to 
weigh what's important, and act accordingly. If 
you love the cel to pieces, and you'd love to 
track down its background/sketch/whatever; or if 
you love the sketch for the sketch itself, chase 
it as far as you care to go, and resign yourself 
to tuna and ramen to make up for it. ^_^ But if 
it's "just nice" to match up missing pieces of a 
set, you might want to let it go. After all, you 
already own the cel. ^_^ You might keep track of 
the bidder's identity, wait a respectable amount 
of time (months? over a year?) and then contact 
them, letting them know of the situation. 
Perhaps, by then, the sketch's novelty would have 
worn off, and they'd be more willing to part with 
it. Heck, you might consider contacting them 
now... but that probably wouldn't go over too 
well, and might be seen as guilting them away 
from bidding, which might lead to bad feelings. :o
(

Good luck!
-Cres