Personally, I love having the sketches that match 
my cels.  I agree with Drac, I like having all of 
the pieces/parts that go with my cel.  Plus I'm 
big on the whole "process" - from rough genga to 
douga to cel to film.

However, I haven't let the absence of a sketch be 
a dealbreaker for me...  Yet...  ;-)  Probably 
because I'm used to not having them for some of 
the animes I collect - NightWalker, Fatal Fury...

Plus, sometimes the sketches are just as beautiful 
as the cels!  Take Allen for example (key cel):