People copy links to images all over, even on non popular sites. It consumes bandwidth when people just hotlink, and it's stealing when they just take it. Might not seem like much to some people, but it does have the annoyances.
Especially on member based websites, people steal other people's pictures all the time. I have people asking me to add a no-right click, but I haven't because like everyone else has said on here, it's easily bypassed.
I was on flickr one day, they actually do a different trick. They have a clear 1 by 1 gif, and then for an image, they place the 1 by 1 stretched out on top of the image.
i.e. something along the lines of
<img src="myimage.jpg" width="250" height="100"><div style="position:relative;left:-250px;top:-100px;"><img src="1x1.gif" width="250" height="100"></div>
Pretty nice trick, but still bypassable by simply viewing source.