...is that the cels can and do stick to that 
paper (quite easily, in fact...they can stick to 
ANYTHING you back them with, even celbags ^^;; 
That's also why you should never keep you sketch 
in the same bag as your cel...), so watch out 
^^;; It's not just 'production mishandling' that 
causes sticking, trust me. You don't have to 
separate all your layers, we're just trying to 
suggest keeping them in bags whilst in books. 
Again, I'll quote Trebby..."I don't trust 
anything that's dyed black is 'acid 
free'." ...meaning I wouldn't trust that the 
paper in the books wouldn't degrade your cels. 
You don't HAVE to do this...it's just a 
suggestion.

Hope this helps!

-Misha