Interestingly the only places I have seen this done for "art" the images are absolute crap anyway.
Heh, so you haven't seen any actual graphic designers do this? You know they do it because they need to protect their work, right? Otherwise their client would just take the work, use it, and not pay. Granted, I don't think watermarks are the best way to protect artwork--showing a low resolution image of it is much better. It's unusable anyways, whereas a watermark is easily removed by anyone who has some semblance of Photoshop skills.
Obviously there are crap artists who do it, but if there are people (idiots?) copying from crap artists, who are you to complain? It's not like crap artists all draw Naruto (or other copyrighted) pieces; some just draw original crap (unfortunate!). And they get copied. (Which is rather sad)
For the record the most profesional art site out there Conceptart.org never does this with images.
For the record, he's not trying to sell his art. If he were, he wouldn't be idiotic enough to post his images in that high of a resolution (without an extremely glaring watermark), get it stolen, and then promptly complain. He's already got a job and isn't looking for another one. That's very different than freelance graphic designers who do need to make sure their work is paid for.
This has gone rather off-topic, from right click disabling to watermarks. :-P Anyways, back to the topic. Yeah, disabling right click is entirely useless. You have no idea if Javascript is even turned on in their browser (I, for instance, always keep NoScript on). I can click on any right-click disabled site without any hassles. Moreover, sometimes right click disabling doesn't even work--I remember trying on Firefox on a right-click disabled site--the alert box telling me not to right click showed up, but the right click menu showed up as well, so I could click as I pleased.
Finally, if you do want to protect images on your site, either don't post it to the internet or use low resolution images as a sample. You are merely making the web an annoyance for users who don't actually want to steal your content, while those who do want to nab your content can very easily do so anyway.