Nahh, at least for comic books Mylar comes in 
thick and thin versions.  I've gotten a couple of 
cels that came with think Mylar bags on them; I 
don't know if they make the thicker/stiffer style 
in cel-sizes.

(Yep, I justify my cel-habit by pointing out that 
they take up less space than comic books.  Of 
course, that hasn't really stopped me from 
*still* collecting the comic books!!!)