I'm definitely with Lizzard on this one.  I like
cels mainly as a "slice of the anime", rather than
just a pretty picture.  A cel with a mismatched BG
is more like the meringue from a slice of lemon
meringue pie slapped on top of the meat from a
slice of shepherd's pie--a combination that leaves
a bad taste in my mouth.  ;-)

I won't display cels in my gallery with unmatching
BGs if I can help it.  Cels with stuck unmatching
BGs are a trickier question.  I wouldn't
intentionally buy one that way, but if the BG is
sufficiently generic I might keep one.  (I suspect
my Sailor Pluto of being stuck to an unmatching
BG, but I just love the cel too much to let her go
or risk destroying her.)

--Atiya