I think Atnas' point is: why completely ruin the artwork with a watermark? What's the point of showing it off if it's ruined like that? Also, why do you care if people post it on other websites?
I personally never got the whole "credit" thing. I know of at least one guy that uses some of my art in his portfolio to increase his chances of getting a job. This is preposterous and funny to me, but I'm not going to start watermarking all my art, just so idiots like him can't claim it's his. And yes, I've seen my artwork turn up all over the place. At one point, an RPG base I made became more famous than I ever will be, I never got credit for that. I made a set of smilies once that's still being used across the internet. When I made them, they were only to be used on one site.
Point is, it doesn't hurt my revenue, so what? I agree with Atnas' grandfather. I do some portrait work, and I've developed a small insignia instead of a signature, that I can subtly work into my work. The insignia is like my seal of approval that I put pride into the piece, instead of a "loud" signature that feels very pretentious indeed (not connected to you btw, a watermark is of course protection, not egotistical).