I never used to watermark, either.  However, 
last year some person actually copied my ENTIRE 
site (the layout and EVERYTHING.. she just 
changed my email address to hers). She even left 
up the part that read "please email me if you 
want to make an offer" ^^;;;.  Needless to say, 
this freaked me out.  I (along with several 
others) bombarded this person with email until 
they eventually took the site down.  

After that, I started putting my email address 
as a watermark on all my cels.  Now since my 
email address keeps changing, I just watermark 
with my name.