Ask any ANY bigtime vintage cel dealer (Disney, 
Warners, etc) and they will hit you over the head 
for framing with glass.  Glass is baaaaaaad for 
cels.  Verra bad.  I wouldnt touch a cel framed 
in glass with a ten foot pole unless it was a 
really cheap cel.  Glass has been known to warp 
cels, cause condensation behind glass, etc etc 
etc.  A lot of reasons cels get damaged when 
framed is because they're behind glass.  Plexi 
may look different, but its a lot better on the 
acetate than glass is... 

Oh and, of course this sounds dumb, but never 
clean your plexiglass with windex.  Thats why 
plexi fogs up so much.  Use brillianize or any 
other plexiglass cleaner.  It keeps everything 
verra shiny ;)

-Karen