As one of those people who religously marks all 
his cels and scans, I figured I would answer this 
one. In regards to someone making a repro of a 
cel you own, whether you water mark it or brand 
it, they can still make a copy of it if they have 
a minimum amount of artist skill. Of course they 
can always make a screen cap and use that to copy 
from so branding your cel makes it even less 
useful in that regard.

The reason I brand cels and why I think most 
people brand cels is either to discourage band 
width theft (people linking directly to your 
images) and/or discourage people from attempting 
to use your images in a shill auction. After the 
first time you see someone trying to use one of 
your cels in a fake auction, you suddenly start 
getting a bit paranoid about marking your cels. 
Lastly, on a more positive note, if you don't 
mind people keeping images of your cels on their 
computer, the brand/watermark does serve to let 
them know where they got that image. So for all 
intensive purposes, it becomes a form of 
advertising.

Catch you later,
Marcus