Well,
IMHO, backgrounds are a plus no matter what.  Im 
a -big- fan of non matching backgrounds.  I buy a 
background for each of my cels, even if it doesnt 
match.  So far, I only have two cels with 
matching bg.  Cels with matching bgs from certain 
series are very very hard to find.  I collect 
mainly FY, and it's nearly impossible to get 
those with the backgrounds, and all the matching 
sketches. I always keep an eye on Celga and Taro 
for the lovely cel backgrounds they offer for 
sale.  To me, it really makes the cel pop.  If 
its the matching one, awesome, if not, so what, 
it's still pretty, and it makes the cel look more 
like a scene than if it just has a blank sheet 
behind it.  And, who'll know?  Most of the time 
you can find a bg out there that's very close to 
the cel's actual bg, or one that will complement 
it even more than the real one would.  We'd all 
love to have the matching bgs for our cels, but 
it's so hard to find them that way.  Why not 
improvise a little?  Even lazer copies are good.  
Some of my favorite bgs are copies from other 
nice collectors!  While others find it ill to put 
a non matching bg with a cel, I think it's right 
as rain :).