> Or are signatures so worthless that nobody 
> cares about this?

Exactly the opposite, I'm guessing.  :)  Someone
who would pay a premium for a cel with a signature
isn't really jonesing for the cel itself.  *grin*
 It's the signature of someone they respect or
admire (or, in the case of fangirls and seiyuu,
lust after... *ducks and runs*) which is worth the
increase in price.

Now, personally, I think a signature "ruins" the
cel, but that's because I love cel artwork and
don't really care for autographs.  If somebody
offered to sell me one of my top wishlist cels,
but with an autograph on it, I'd most likely
decline.  But hey, if that signature was on a
different layer, there's no longer a problem.  :)

I think it's a good compromise.  Just my
two-yen's-worth, of course -- your mileage may vary!